I can't pick 100% of gay guys with gaydar, but I'll bet that when I do pick one, I'm 90% accurate. I am convinced that patterns of eye blinking are the most obvious way to tell. I know that studies on eye blink rates have not shown correlations, but I nevertheless feel that in my personal experience, when I observe a certain pattern, 90% of the time it's accurate. I would describe it as a slowness in the closing and opening of the eyes, not increased or decreased blinking.
Vocal inflection and mannerisms are also very accurate. Studies I remember seeing show that most people can identify voices as "gay." Although the general identification is not accurate, it does show that certain inflections are considered "gay," and the presence of these inflections in certain situations, combined with mannerisms, makes it easy to tell. When a guy in a society like NC does not attempt to hide obviously gay inflections, and/or does not attempt to hide mannerisms that are effeminate or indicate physical attraction towards other men, it's a pretty good sign. In environments where such behavior is more common and less likely to be condemned (such as in Greece), I don't think this applies.
The most foolproof way is to observe a guy talking to another guy friend. His posture, eye movement, inflection, etc, will reveal whether or not he sees the other guy as a sexual possibility, whether or not he is attracted to the person. Even if he's a very masculine jock type, the way he looks at and orients himself towards guy friends subtly reveals his orientation.