Hi. Personaly I look for the way guys (I notice guys more often, not so much women for obvious reasons) look at other people of the same gender. The way people walk (especially if two guys jwalk just a little bit closer than I'm used to see), talk (especially the voice tone and way of saying certain words), stand and the way they dress. I know it's shallow and superficial, but besides my boyfriend I know very few gay people. Some more flamboyant guys are easier to scope out, but most look just like any other guy.
I think my gaydar works somewhat well, but as far as I know I fly so low that no one considers me gay. I say this because all the people who know I'm gay never figured me out as being gay, except those who hung around me and my boyfriend a lot while our relationship was still in the shadow for everyone, which was roughly about a year and a half.