At your discretion a partial dose of poison may have (reduced) effects.

The DMG (257-258) distinguishes between different kinds of poisons and how they take effect, by contact, inhaled, by injury or Ingerido (DMG 257):

Ingested. A creature must swallow an entire dose of ingested poison to suffer its effects. You might decide that a partial dose has a reduced effect, such as allowing advantage on the saving throw or dealing only half damage on a failed save.

Handle it at your discretion. Maybe you want to use the Variant rule on the same page (DMG 136) that makes identification generally more difficult and adjust it by making it explicit that tasting potions is dangerous or bears no effect at all.

The narration of poisons are mostly at your discretion as well, you define whether they are tasteless, smell-less, indistinguishable (for that matter you also narrate the smell and taste of magic potions and you may indulge in that healing potions taste foul or like soap), etc. How identifying potions should work gives narrative techniques.

Contact poison only works on exposed skin by RAW, but as a DM you are free to change that (you should inform your players when you do that and before it has negative consequences for them) your character's lips and mouth technically is all mucous membranes and not skin. But I'm not arguing that point, if you think it is skin, then contact poison affects you while tasting. These mucous membranes of humans are even thinner than skin and in that way, realistically speaking, may be affected by contact poison (DMG 257):

Contact. A creature that touches contact poison with exposed skin suffers its effects.

The uncommon magic item (potion) Poção de veneno, (DMG 188):

This concoction looks, smells, and tastes like a potion of healing or other beneficial potion. However, it is actually poison masked by illusion magic. An identify spell reveals its true nature.

tastes like the mimicked potion by illusion magic, that is it tastes like whatever potion it is masked as (which you can narrate as you like). Which is contrary to its true nature - revealed by an identificar spell. So the actual taste is not that of the mimicked potion (which you can narrate as you like).

The tasting of a Potion of Poison replicates the look and smells with illusion magic. It does not replicate the healing effect or other benevolent potion effects.

A Potion of Poison tastes/looks/smells exactly like a Potion of Healing.

If you're talking about the Poção de veneno artigo:

This concoction looks, smells, and tastes like a Potion of Healing or other beneficial potion.

PCs that taste it will think it's a harmless Potion of Healing, and get a nasty surprise when the situation is dire.

It's a trap!

What you're describing is technically a trap (except in the case of the potion of poison, which would need to be fully drunk in order to have any affect), and you should treat it like one as a DM. You need to decide as the DM whether they detect the trap and whether they trigger its effects. If they trigger it in a cautious way, you may consider reducing the DC of the saving throw.

I should clarify, it doesn't have the intent of being a trap, what I'm saying is that you can treat it like a trap in this situation. I've come across this type of things a number of times as a DM, the players want to do something that is stupid (my players think it's funny to do stupid things, and they're right) and I essentially improvise a trap situation for them and attach an appropriate DC for them triggering it based on what exactly they are choosing to do.