An obvious way to catch someone you suspect as a closet dumbass is to ask them about something that you already are well versed on and watch them stumble as you contradict them when they're wrong.
(Eric walks up to join the conversation)
Eric: Yeah me too. Im a huge fan.
Rob: No you're not. You've never even heard of Dio.
Eric: Yeah I am. In fact I just saw them in concert last month.
Josh: Last month? Horseshit! Dio died last year.
Rob: Yeah really, Eric. You're such a closet dumbass!