Flaming hair, tight jeans and the way they talk. But sometimes I get this confused with being English and/or stuck in the 80's.
I have shit gaydar. Unless the guy was on his knees sucking a dick I would't know if he was straight or gay.
OBVIOUSLY there are some instances when they are obviously gay. But then again I have been SO WRONG in the past about that before.
It's more about instinct than anything else, and yes, I'm usually right.
Apart from the obvious effeminate mannerisms and speech, being overly well groomed, being overly polite or animated, never mentioning women or sports in conversations, or a kind of uncomfortable awkward forced masculinity like Tom Cruise or George Michael (when he was a star).
Guys where my gaydar failed miserably - Anderson Cooper, Gareth Thomas, Robert Reed (Mr. Brady), Barney Frank.