If the characters were presented with some cubes of cooked meat, what kind of check would be necessary for them to make in order to determine what kind of animal or creature it came from.
For example, if it upon cursory examination appeared to be ham cubes, but was really human meat cubes, what skill or knowledge check would be relevant to tell the difference? Profession: butcher to tell it's not ham? Knowledge: nature?
For some more context, I want the players to find a magic bag that fills with cubes of cooked meat that appears to be ham, but is actually cubes of human flesh with some way for them to eventually find out that they've been eating people "all this time" without knowing it.
Update: If anyone is curious, as per the answer I've accepted, I've decided to go with these perception rules for it:
DC 5: you think it's ham
DC 18: you know it's not ham, but you're not sure what it is
DC 28: you know it's human.
Any character that has knowingly eaten or smelled cooked human meat a +20, and any one with a butchering experience a +5 towards the DC 18 check.