Easy way to tell.
If they complain about the ten billion TPS reports they filled out and explained how boring their career was, they were probably in the CIA. Bonus points if they're old codgers and they whine at length starting with "Back in my day..." Extra bonus points if they sound disappointed regarding the fall of the Berlin Wall. If they're rabidly misogynistic, it's a definite.
If they claim to be CIA assassains or try to sound like James Bond, they aren't.