I used to think I had really good gaydar, but after a good chunk of life and knowing a lot of gay people (many of them friends), I think that gaydar is somewhat of a myth. Even a lot of the gay guys I know are wrong half the time, which means you might as well have flipped a coin.
The way gaydar is supposed to work is that you pick up on various nuances in the way a person looks, talks, acts, dresses, or moves that would tip you off that they are gay. Guys with effeminate movements and voices, very short haircuts on women, a masculine attitude from women, etc.
The problem with all of this is that there are often major exceptions to the above. I've met men that other gay men SWORE were gay, only to find out they were happily married with kids. Now, whether those same guys are really living a lie because they don't feel comfortable coming out to their families (maybe they fear being ostracized) is another question, but for all intents and purposes, publicly they are straight, and if they don't engage in gay activity, can they really be considered gay? Interesting topic for another thread there. The point is that you really can't apply universal rules to things like this -- heck, even some men who have sex with other men don't consider themselves gay or bi, just kinky/macho.
Anyways, there will of course be some men and women who are very obvious and are easy ones to spot, especially if they are out of the closet and proud of who they are. I would say they probably don't count when it comes to gaydar, because it would take the most naive or blase' person in the world to miss this.