This is one of the things I love and hate about being gay. I love it because it adds an extra layer of tension and excitement to every encounter with a guy I'm attracted to.
I hate it because 50/50, 40/60, 37.5/62.5 % straight/gay guys can often have a huge charge over you even thinking they are gay.
One of my solutions is to be out. Out everywhere, out all the time. If I'm out to the world, then those guys struggling with their sexual identities, or too scared to be out themselves, have opportunity to make the first move.
I've had friends who are straight to the world, but after months of knowing me, finnally come out.
Being out all the time is a strain, and I've paid a price for it, in lost friendships, homophobic conservative christian nut job bosses just looking for an excuse to fire me, family who don't want to even look at me, etc, etc.
But the blessings are huge. All of my neices and nephews have "the coolest Uncle in the world", and a change to make a difference in this generation.
But that wasn't your question. You asked how you can tell if someone is gay, and the answer is, unless they tell you point bank, you can't. Cool shoes and spiked hair, marital status or lack of a girlfriend, the colected works of Patsy Cline and Barbra Streisand, or any of the million other indicators are useless, because we live in a society that tells boys from a very young age that being gay is risky business. Go to Craigslist on any given day and find the posts for "MWM looking for bi guy for hookup", and all the other ways men tell themselves they are not "really" gay.
Just asking someone out on a date is scary and exciting all by itself (at least for me!), It is a real challenge to have to wade through all the bullshit jsut to get to that point.
Oh, and that bit about "an obvious, typical gay guy"? I was that guy for about ten minutes after I came out to myself, because I thought that's what I was supposed to look like. Yuck! Now I have gay friends who acuse me of being "straight acting". Just because I dress like a lesbian, without the spiked hair and cool shoes.