Well unless the guy is "flaming" with jestures and a lisp it can be hard to tell for sure. But with those who aren't stereotypical gay, well who are their friends and who do they talk about most? If they are predominately guys, then that gives you a good clue ... The key is to use your intuition, give it a little time (a guy that is straight might send out a few signals that taken individually could be interpreted as guy, but that could be a false positive) and you will have a 95% certainty of knowing.
When a guy is fashionable and is flamboyant he can be gay and when he is seen looking at other guys than for sure he is gay. You are going to have to ask the guy than to see if he is or not. If he will not tell you, well stay away from him until you know for sure he is straight or bisexual. I have no idea whether you like men, women or both, so tell me,