Talk to the guy in red outside the hotel on fountain rock. He's only available after you've completed Gangland.
It seems that the hotel has a problem: some of the prostitutes are in fact transvestites. There are three, four, or five of them in total, and it's pretty obvious which ones they are. When you've found a candidate, get in close and press to make Tony give him a kick to the crotch. Make sure you don't kill anybody.