Comes from the movie "Hook". Robert Williams, as Peter Pan, tell Tinkerbell (played by Julia Roberts) that he does not believe in fairies. Tinkerbell then faints and acts like she's dying. In order to revise her, she tells Peter to "Clap if you believe in fairies." A "fairy" is another term for a gay person, so it works.
If someone says this to you, clap your hands if you agree that the person in question is gay. If not, just say he's metrosexual.
"Clap if you believe."