You make the advantage roll in secret

In situations where I don't want players to know they have advantage/disadvantage for any reason I simply roll a d20 for them myself and work out the results. This usually works out just fine because the players do not know if I am simply rolling an opposed check, for example.

I also make it a habit to roll d20s behind the screen for no reason which also helps with the ruse.

This does require that you either know their modifier or keep an eye on what their d20 result was and calculate it from there.

There is, however, one issue I have found with this method: it messes with abilities that allow players to reroll before knowing the results and things like the Lucky feat so it is not an universally applicable solution.

This is precisely what the passive versions of skills are for

The recommendations for when to use passive checks are:

Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

To calculate the passive check value use this formula:

10 + all modifiers that normally apply to the check

Then compare that value to the DC for the check.

There are modifiers for advantage (and disadvantage) as it interacts with their passive perception.

If the character has advantage on the check, add 5. For disadvantage, subtract 5. The game refers to a passive check total as a score.

When I DM I get the players to fill out a character tent with their ability scores, proficiencies and AC that I can then refer to if I need passive check info.

But isn't not asking for a roll suspicious in and of itself?

Not necessarily. In general you should only be asking for rolls when there is a chance for the characters to succeed.

It's entirely possible that the ranger is the only one that has a chance at succeeding at the check and thus it would be inappropriate to ask other players to roll.

This points to a deeper design issue for your plot

From your description, you have centered part of the mystery on 'what happened' knowing that 'what happened' falls squarely into one of your character's areas of expertise. It's reasonable that such a character should have advantage on the perception check, and will thus ruin the mystery.

The design issue is that you have hinged an important part of your plot on concealing something that can be discovered with a single roll. You are wanting to conceal information from a character whose specialization makes concealing this information very difficult.

It's hard to conceal a brutal raid by giants. For your specific case, you might take a look at why they did it. Was there some deeper motivation behind it? Was there something strange about the Giant's behavior that does not mesh with what the Ranger knows about his favored enemy? It's not the things that fit a character's expertise that make for the best mysteries, it's the things that do not fit.

If you hinge your mystery on things that the characters figure out because of their expertise, rather than on details you hope they don't find despite their expertise, you will make better mysteries and engage the players in those mysteries. Spoilers will not come from a good roll on strong stats, clues will come from it instead, especially clues that fly in the face of what one would expect to find. This way, you are disappointed when they fail to spot something, rather than disappointed when they succeed.

I'm not going to try to suggest specific changes to your plot, so I'll simply answer your question directly, based on what I've suggested above:

Make the plot happen 'because' of his advantage, not despite it.

Give up the idea that you can create a mystery around 'what kind of creature did this?' and up the mystery game with details that only his expertise can reveal. Hinge it not on 'what', which can be answered with a roll of the dice, but on the 'why', 'who' and 'how'. Maybe the giants were accompanied by a Dwarf ally? Maybe there was something weird about one of the giants tracks? Maybe the raid was precise, but the giants are of a chaotic sort? Maybe the raid was exceptionally brutal, but the giants in question usually prefer to get in and out with a minimum of collateral damage? Whatever you decide, it feels more engaging if the player feels that their advantages are advancing the plot rather than feeling like their advantages are frustrating you.

You tell them

"You see scattered boulders that seem to have come from nowhere and large imprints in the ground. Giants have been here!"

Favoured enemy to me is like passive perception, you don't need to ask about something to recognise it.

Once they have been told about the giants they still have to make the tracking check, so they only find out so much.

In the general case, the suggestions of using passive checks (preferred) or secret rolls (eh, but okay) are good.

But I think in the específico case, where there are literally giant footprints, it's not going to take a check to guess that yep, these are giants. (I'm picturing a scene with the ranger carefully inspecting and measuring this gigantic boot-print, and then dramatically announcing "Giants!", and then everyone nods very, very slowly while exchanging significant glances.)

Favored Enemy is not usually a very powerful ability and is super situation-dependent. As a DM, it's nice when I have a situation where a player's niche feature aligns with the story. Sure, the main mechanic of this feature is advantage on specific checks, but, also: "... you have significant experience studying, tracking, hunting, and even talking to a certain type of enemy." As a DM, I would play up the fact that the ranger is sure that these are giants based on their class feature before even asking for an ability check. This isn't a spoiler — it's rewarding the character.

But that's not all — sure, they are giants, mas o que giants? Looking on D&D Beyond's monsters page Entendo dezenas of listings for giants, even discounting the entries for specific NPCs or less common settings. Now that the ranger has pointed out the signs, everyone can tell that these are giants. But your ranger's special expertise gives advantage in figuring out more — frost giants? hill giants? cloud giants? ogres? trolls? terrível trolls? Essa is what the check might give.

Use the rules for a passive check, but with a twist!

Como mencionado nesta resposta when you're using passive ability checks you add or subtract 5 for advantage/disadvantage, but I disagree that a passive check makes sense here.

If you're really worried about telegraphing anything to your players then ask the ranger to roll as normal (not with advantage and not as a passive check) but mentally add 5 to what they say (or lower the DC by 5, same thing)! This way you have in a sense given advantage which is mathematically the same as what the rules for passive checks describe but not telegraphed anything.

Roll a second dice behind your screen

If you want the ranger to unknowingly roll with advantage then this is the simplest solution.

When the ranger makes his check, roll a second d20 behind your screen, then take the higher of the two rolls (his roll and your roll on his behalf) and give him information based on that, not necessarily the roll he rolled.

Your players will know something is going on when you roll a check but they won't know exactly what. If this is likely to be problematic at your table then roll a few d20's in advance of the session to use for this purpose and keep a note of the outcomes.

I think you may be looking at this wrong if I understand the issue correctly. It doesn't sound like the rangers advantage would really apply to them knowing a giant was here. In your OP you stated: "one of the major clues is the fact there are dozens of boulders scattered around the town that were dropped from a cloud giant castle from a great height. As well as footprints that were left behind when they eventually descended from said castle."

The key part being "As well as footprints that were left behind"

Knowing that at least one giant was "here" sounds like it would be obvious and have nothing to do with the rangers expertise. Every character present would clearly see the huge footprints and know that some form of giant like creature was here. It is determining details of what those footprints reveal, that would make use of the rangers advantage. The real question is can they figure out what happened de those footprints and other clues. (a giant could have lived here or been visiting for trade for all they know)

Significado: If the ranger decides to try to study the tracks and figure anything out from them, they could realize any given number of things depending on their roll (which would have advantage because its obviously a giant print of some kind). Otherwise they just know that some kind of giant was here at some point, doesn't mean it did this...

Footprints could possibly reveal things such as:

• what kind of giant made the footprints
• was it one giant or many
• how old are the footprints
• add a perception check to notice if the disturbed earth around them is about the same state as the disturbed earth around the boulders? (IE is one dried out while the other fresh, were they disturbed about the same time?)
• they were disturbed about the same time? do these types of giant typically use large boulders to attack
• they do? hmm maybe our visitor(s) weren't here on friendly business... which way did they go when they left?

etc., etc.

Just a thought at least :)

If you want to play it as that it is not obvious that a giant was here, but just that something big was here... you could have them roll with advantage because it looks like a giant print, and if they fail both rolls, simply say "you know giants pretty well and while you are not sure what made these prints, you do not think it was a giant", or something along those lines.

Simulate advantage by changing the DC. This way there is now way to know unless they succeed. There is a bit of math to do this correctly, and it won't be exact, but a good rule of thumb for normal checks is to subtract 5.

For example a DC 10 check with no bonuses is equivalent to a DC 5 check with advantage. A DC 15 is equivalent to 8.75, which I would round to DC 9 for advantage. These numbers can change based on their bonuses, so a DC 15 for someone with a +5 bonus with advantage is the same as a DC 10 with +5 and no advantage (as opposed to the 8.75 from before). Getting the exact number exactly right isn't grande important so I would just subtract 5 and not bother with the calculations.

If they fail they just think they rolled normally. If they succeed, you can explain why it was different for them.

Invent another reason to give an advantage

Advantages don't stack and as a DM you are free to invent circumstances where the ranger would have a different advantage that would apply. Depending on circumstances there could be numerous other valid reasons to give an advantage, for example:

• Crossing country went faster than expected and tracks are very recent.
• The dwarf in the party recognizes that the rocks are not from this area.
• Ranger's animal companion recognizes the smell.
• After asking to describe how exactly they proceed with the search, they "coincidentally happen" to be looking for exactly the right clues.
• The weather today is exceptionally nice.

This approach doesn't reveal the plot twist, while being mechanically identical, as well as makes players feel good because they still gain advantage instead of being suspicious about DM plotting behind their backs.

In any case you shouldn't deceive your players longer then until the end of the session and upon reveal you should explicitly tell ranger that he and his skills were the reason they had the advantage.

Additionally if the player starts guessing the reason they have advantage all the time is because of giants even if they have failed rolls so far don't hesitate to confirm their suspicion and congratulate the player on solving the puzzle.

In my mind, the Favored Enemy bonus should be just as useful in ruling out a source as it is in determining it is positively that source. If you're so intimately familiar with something that you automatically have advantage when recognize it, then you should be able to just as readily recognize when something is not that thing too. That is, your ranger should get advantage both to say "This was definitely giants." and to say "This was definitely not giants." The second statement doesn't tell them what it was, but it's still something (and a benefit from their class feature).

Implementing this means establishing a precedent that when the ranger rolls Survival (Wisdom) to determine what made the tracks they always roll twice and report both results. As the DM, you then report information to them like so:

1. Both fail DC: They don't know what made the tracks; could be giants, could be something else.
2. First roll fails but second succeeds and giants made the tracks: They know giants made the tracks.
3. First roll fails but second succeeds and giants did not make the tracks: They know giants didn't make the tracks, but don't know what did.
4. First roll succeeds: They know what made the tracks.

This does mean more die rolls, but it's only for cases where they need to identify what made the tracks. That is, just the first Survival roll needed in a particular encounter. On subsequent rolls you can cut back the number of rolls based on their determination in the first.

Of course, in order for this to work, you have to apply it consistently, i.e. anytime they come across tracks whose source is unknown initially. As a result, it may be too late to not tip your hand in this case. One thing you might try, though which might still give things away, is to rule on the first die roll normally and if it fails then ask the player for a second die roll "to rule out giants." In this way the second roll is framed as a benefit from their class feature, but because you haven't explicitly drawn the "roll again" = "giants" connection. Players might still make this connection, especially if you've never done it before in similar situations, but if you establish the precedent and carry it forward, they are less likely to draw that conclusion when this situation comes up again.

Another thing to think about in deciding whether or not this will work for you are whether you've actually given wrong information on a (spectacularly) failed check before. If you have, then this first time implementation can be phrased as a correction for that (especially useful if the first roll is 1), thereby reducing the chance of tipping your hand.

Further, there is the possibility of deception. How would an attack faked to look like a giant attack look to this ranger? If that's something that's come up in your game before, this can be framed in terms of that, and again reduce the chance that you'll tip your hand.

Just be ready to keep this going forward, as it is an advantage for the player to be able to rule things out (albeit one I think they deserve) and they will guard it jealously.

Feign the need for a second perception check. "There are two different things to notice, here. Roll for each." This way, the player gives you the two totals (which you need to adjudicate advantage) and if they miss both they will never know what happened.

If they hit at least once you can reveal that it was secretly an advantage roll.

After the first or second time, the players might start suspecting something, especially if other players ask for the same check (this actually happened at my table).

So only use this sparingly, or have some other irrelevant flavour detail that you can give players for their second roll.

if the player asks to take a look at the boulders or tracks left behind, I would have to ask them to roll the check with advantage.

While an argument could be made in favour of this, I don't think it's always true, especially considering that the ranger will not know that it was giants.

In my opinion, the Favoured Enemy feature is not "if you track something and it happes to be your favoured enemy then you get advantage" but "if you explicitly trying to track your favoured enemy, then you have advantage".

E.g. for giants, the ranger knows that they leave big footsteps, how a giant would walk in a certain terrain, what sort of tricks they use when they don't want to leave tracks etc. Similarly with attacking; if it's a giant hiding under a blanket, the ranger wouldn't be able to utilize her knowledge of a giant's weak spots to gain advantage as they don't know it's a giant (See Quem deve saber / informar o Inimigo Favorecido do Arqueiro?).

Of course, it's a bit of a stretch to assume that a person seeing giant footsteps will not assume that it's a giant, let alone someone who is an expert on them. But perhaps the tracks aren't clear, other creatures could throw boulders etc: that's up to you. It would make sense to allow for a perception check for the ranger to realize it's giants and only afterwards to have advantage for tracking them; but if the whole plot relies on them not immediately realizing it's giants, it doesn't make sense to allow that roll (but as MarkTO said it does sound like a weak point of the plot).

How about you say that he is allowed to roll as if it were advantage, but you will decide, without telling him, whether you take the second roll into account depending on whether his skill applies or not.

Ofcourse, this means that you will have to do this on other occasions as well, to avoid shifting the meta from the problem with advantage rolls on Favoured Enemy, to meta on this "trick".

(Disclaimer: I'm very noob at D&D so I have no idea if there's anything in the rulebook, or some common practice for this.)