I agree with everyone else... sometimes people can be "obvious", or sometimes there can be hints that might sway a person to making a conclusion, but it is easy to be wrong. For example, not all camp guys are gay. Not all butch women are gay. Funnily enough I was once told by a friend at college that I wasn't gay because I didn't look like one...

It's interesting to think really... I wonder what I come off as...

~Edit-
I had another thought. People tend to use confirmation bias as well...
Let's say that there was a boy. He once played with a Barbie doll.
In scenario A, he grew up and was straight. The Barbie doll is forgotten.
In scenario B, he grew up and was gay. The Barbie doll is used as "evidence".
Not just with other people... those of us that find ourselves to be non-hetero can often find apparent evidence in our pasts to match our identity if we look.