Mate, If I knew I think I would be rich. Seriously, ALL gays are different and while the 'stereotypical gay' is still somewhat relevant, it by no means applies to everyone. Take me as an example. I hate shopping, I don't speak with a lisp, all I do with my hair in the morning is dry it and I don't have floppy wrists. (ok, I admit that was a poor description of a stereotypical gay person). But all I have in common with the steriotype is that I like guys. If you knew me quite well and looked for signs that I was gay, you most probably wouldn't find any. This is also the case for a lot of guys. These guys are usually labelled 'straight acting'. Unfortunantly, even the guys who seem to fit into the stereotype or appear to be 'camp' aren't all gay. So even if you think someone 'acts gay' it could just be their vibrant personality. So I'm affraid even if you look for specific signs, there still isn't a sure-fire way of knowing ones sexuality. Having said that, you can sometimes check their reactions to certain actions, such as leaning against them, flirting with them etc, and see how they respond. Aswell as actions, you can bring up certain gay topics and see how they react. Again, this is no sure-fire way as some straight guys play along anyway, but it had been the most helpful indication for me.
why wouldnt you ask? Hipotetical (I know I missed an H somewhere) situation: you: 'Hey, i think you are hot... are you gay?' possible answers: him: 'yeah, and btw you are hot too... come here to this dark conrner... *makes evil face*' him: 'hmmmm, no, but thanks... I am flattered' him: 'yeah, and I am taken/have a stomachache/not interested/<any other drop line>' him: 'NO! and I AM AN UNDERCOVER AGENT FROM POPE MAZINGER Z AND WILL SLAY YOU NOW!!!'
Anyway, irony aside. Most people are so sure I'm straight they feel comfortable joking with me like touching or almost kissing. And most of them would be outraged if they knew I had sexual fantasies with them. Anyway, there is no sure way to tell, some people call it a gaydar... the ability to tell if someone is gay or not... I don't know... I am not the best one to give you an advice here. If he's out, at least to himself, and you listen very carefully to everything he says, the way he speaks, the things he says, how he says them.