In D&D 5e, I can't find a skill check for identifying monsters. (Other than Arcana for planar monsters). Can PCs identify monsters, and if so, what skill should they use? (Or is it just a Wis/Int check?)
It's almost certainly going to be Wis and Int Checks. The question is which skill proficiencies will apply to those checks depending on what their origin is.
This is generally the Purview of the Monster Manual and DMG, however, Mearls has said that more of Basic D&D is will be released this coming week. We should know a lot more when Basic D&D expands.
As a DM, my rule of thumb is very simple.
If they have encountered the creatures before, then they should be able to identify said monster. One usually doesn't forget monsters they have encountered.
If they speak the language, they should be able to identify the monster. Knowledge is power.
If they are part of that race, half-orc / half-elf etc... they should know what the monster is.
Lastly, if they have dedicated off-adventure time to the study of races and cultures and the monsters race has been in their studies, they would be able to identify the monster.
If not, so sad. Previous knowledge from other characters played does not allow the characters to have insight. Common problem. A dog appears beside you, attacks and shortly thereafter, disappears to reappear a few feet away. Player yells "Blink Dogs, I know how to kill them" How did they know it was a blink dog. The players have to play their current character and the DM has to ensure that they do. Makes the game a whole lot more fun.