Any (usually) white American male that attempts, sometimes successfully, to convince everyone around them that they are not a bro, despite a certain and varying penchant for decidedly bro-like activities or beliefs; frequently using sexual experience to demand respect from one's friends, expressing a desire to fight others, an enjoyment and subsequent short-lived obsession with whatever mainstream rap song is currently "hot", open objectification of women, using ability and ever-readiness to drink heavily to assert dominance and respect within a social group, etc. are all telltale signs of a Closet Bro's true nature.
Note: Closet Bros know on some level what they are, and are afraid to admit, perhaps even to themselves, their bro-tendencies. They guard themselves against reality by vehemently proclaiming a hatred for bros; this only serves to aggravate the situation, making their bro-ness even more painfully obvious to their peers.
Mike: Hey man, I heard you were hanging out with Vanessa. How's that going?
Kyle: Dude, I hit it and quit it. That bitch don't mean sh-
Mike: Closet Bro! You!
Dave: So Kyle was giving me a ride home, right? And we're sitting at a stoplight, when all of a sudden he turns on the radio and starts blasting a T Pain song. I just opened my door and got out.
Paul: I always kind of suspected that guy was a bro.