The rule of thumb for identifying a creature is a DC of 10 + it's CR, so Ate is probably well aware that there is such a think as a Mimic that has some basic shapeshifting abilities.<br/><br/>So as a rule of thumb if you have the appropriate knowledge skill and taking a 10 would beat 10 + It's CR, assume your character is at least aware that it's a thing. Whether or not they have any practical experience with it is an entirely different question.