I agree with FNMM on this...there is no magic formula to figure out if someone is gay. Who you hang out with, how you dress, how you talk, etc...none of those are real indicators.

For example, two people I thought were gay when I was younger turned out to be straight...married women and had kids. Me on the other hand, everyone thinks/believes I'm straight...I'm married and have kids....but I've been gay my whole life. No I'm not in denial...I've always accepted my homosexuality. But when I came out to some of my friends...they were all surprised that I am gay.

Be supportive of whatever your friend is. Don't assume...communicate.