My suspicion is that Bok has too much generalized intelligence for this, and that kind of a check is going to be looking at a whole bunch of factors. Bok isn't smart, clearly, but Bok is also capable of some level of decision making given that he talks to humans and bonks skaven. That requires being able to look at a creature, assess, and then make a judgment call about friend/foe based on known information.
If Bok makes judgements like this, which the evidence points to yes, then it's falliable.