First of all welcome to JUB.
I guess it depends on how you present yourself. Some gay guys are just totally toned down or don't fit the "stereotypes" that it's hard to tell if they're gay or not.
I also welcome you to JUB!
As bokan800 states about gay guys being toned down or not fitting the stereotypes that you see on TV. He's totally telling the truth. If you seen me with my straight friends, you'd think I was straight too. I don't have any fashion sense. I like getting dirty. I watch sports (sometimes). In fact the only thing that gives me away that my friends notice is that I don't talk about girls. You know when your surrounded by a bunch of gay guys and a "hot" girl comes on tv or walks by on the street....they do there thing. You know, "I'd fuck her", "look at her tits" etc. I don't talk about guys that walk by because then all I would hear from my friends is "Oh man...we so did not need to hear that!" LOL (though I will do it sometimes if they crossed a line with a certain girl)
Anyways now to your problem....
It's easy, yet not so easy to solve. Straight guys, I mean straight guys that are 100% totally comfortable with there sexuality are going to be a bitch to figure out. They will flirt with you, joke with you etc. BUT the one thing that straight friends will not do is touch you in certain places. If they hug you or pat you on the back or bump into your shoulder, put a hand on your shoulder etc. There just being friendly. A gay guy that likes you well, he try something different. Look at how straight guys hit on girls for example. We do it the same way.
1. They will isolate there target from the others (ie you.) so that they can talk to you one on one without other people taking away your attention.
2. They will get friendly....way more friendly then normal straight guys. Their is boundaries. If a person that likes you alot....they start touching you alot. They make eye contact. They will go out of there way to keep your attention at all times.
3. They invite you out to do things with others at first and then slowly start to exclude them for private time with you.
There are more signs but I can't think of them right now.
Now if your dealing with a gay guy that likes you but he's really shy or afraid that you won't like him or he doesn't know that your gay (I don't know how you present yourself in public) then you could always (after getting to know the guy) ask about girlfriends or something that might tip you. Now then if he lies to you then your screwed. But then again, you don't want to date a gay guy that lies to you now do you? (he might be lying because he's still in the closet...but nothing is going to happen until he is ready so why waste your time. Unless he says he wants to date you but he doesn't want people to know that he is gay. Then you have to figure that one out on your own....using your own morals)
I hope this helps somewhat and good luck!
I agree with Lex. You can never have to many friends!!