Como posso avaliar a força dos meus PCs no Legend?

Valixes here - Legend developer (John Watters) and GM - and I'd like to continue on the previous response, more along the lines of what optimization means in Legend. In other systems, optimization is on the personal level - while in Legend, what makes a party truly powerful is its synergy and combined capabilities. So while a character with I am Not Left Handed, or Discipline of the Serpent, may not actually have "EL" equal to their own level, but when their Swashbuckler buddy is close enough to give the crippled baddies a beating, the contributions of the enemy being prone or/and having reduced AC become a lot more accentuated.

That kind of swing that you are describing in your encounters - where they are either too strong or too weak - is likely due to a lack of a dedicated healer or protector character (Since healing is an extremely viable role in Legend, unlike in most tabletops). Thus, I suggest challenging your PC's with debuffs rather than powerful damage - look into Path of Rage for it's 3rd circle for making the group [Shaken], Arcane Secrets and Tactician Spells for their AoE debuffs, and other methods to challenge the party by reducing their power rather than meeting their power with something overwhelming.

EDIT: Additional information. Try not to press action-deprivation conditions liked [Slowed], [Dazed], [Stunned], or other such conditions, upon your PC's. These conditions are nasty, and can change the tide of battle in the blink of an eye. On the flip side, if your party has these conditions, work either around them or with them - and create the [Encounter] with the assumption that you want to ignore them (With [Immunity]'s or buffed saving throws), or that you want to have the enemies be strong enough challenge even if those conditions are applied. Always be wary at targeting bad saves, and when you do so - base the balance of the [Encounter] on the assumption that they will fail. On the fun side of things, watch out for tracks that single out an enemy and ruin their day, like Discipline of the Serpent, because when used against a PC they'll feel as if they are being singled out and screwed. Watch out for using Path of Destruction against melee-characters that make heavy use of SLA/Su's/Spells, because the scratch damage they will take from it will add up. Also watch out for using AoE against parties without ways of mitigating or healing damage, and on the flip side if your party has healing capabilities, AoE damage will be almost necessary to keep up pressure on the party.

A run-of-the-mill fight is typically 2 less than the PCs party encounter level, or EL. You calculate the EL of a given party by the sum of the number of creatures and their level, or just by the creature's level if it's a party of one; a trio of goblins, each at level 1, is thus an EL 4 encounter and would be a pretty standard challenge against four-person adventurer party whose members are at level 2.

EL-1 is tougher, equal EL would roughly grant a 50% chance of the players claiming victory, EL+1 is really hard, and EL+2 should almost never be done if you want your players to live. That's our guideline, subject to waver just a tad under different levels of optimization by either party. Also, remember that mooks and myriads are weaker than your average full-fledged character; it's tricky to enumerate how much weaker, so for now just think of them as "easy mode" for a given EL.

Now, while the gap between optimized characters and non-optimized characters is smaller than that of D&D 3.5, there are definitely some things to keep in mind even when taking into account appropriate EL. I'll have that drafted up later.