Unless they are wearing girl clothes and walking/talking like a girl, I have absolutely no clue which guys are gay. It shocks me everytime I find out.
Statistically, homosexuals make up less than 2% of the population. In some states, homosexuals are not well received -- so they do their best to hide their orientation. Some homosexuals in such areas just pack up and leave: they go to San Francisco or some other big city where they can "be themselves". The end result is that some locations have much LESS than 2% of the population being homosexual -- and the ones that are there do their best to be invisble.
The net result is that some heterosexuals grow up in areas where homosexuals just don't seem to exist. Spot a homosexual? People wouldn't know how!
But me? I lived in Boston, MA for 20 years. I worked in journalism. I worked in academia. I worked in the arts community. Let me tell you: I can spot a homosexual real quick. I don't say my gaydar is perfect, but it's pretty well honed.