It's like Spiderman and his Spider sense. He can just "feel" when there is danger nearby. Gaydar allows you to "feel" when there is gayness nearby.
Usually the gay/bi ones are always the good looking ones. ie: Nick Lachey, David Beckham, Channing Tatum, Ryan Carnes, Chad Michael Murray
and usually the ugly one's are hetero.
ie: Dick Chaney, George Bush and Bill O Reily (sad isn't it)
Jeff: Dave? he's gay, my gaydar pinged when i saw him
Kate: ohh thats too bad. who else is gay?
Jeff: Your husband
Kate: Noooo you lie you fag!
Jeff: I slept with him so take that you fat bitch!