Ben, Kevin, & jaRED all have some good points.
Gaydar isn't one technique or one sign to look for....It's a combination of lots of little things to keep an eye out for that can give a strong indication as to whether someone is Gay or not.
I've always found that the 'eyes don't lie'. As jaRED said, if you can observe where someone is looking (without them knowing they are being watched), you can get a real good indication whether they are gay if they are staring at cute guys or their asses for that second too long. Also, it's pretty obvious if they lick their lips at the same time .
While some people have natural Gaydar talent, others have to continue to hone and improve their gaydar with time and practice.
Observe and peoplewatch...your Gaydar will then be the best on the block.
And as Ben said, don't confuse Wishdar with Gaydar. Cause you can't make a guy gay just by wishing for it hard enough
Take Care