There is no definite way to tell if someone is gay.  If they don’t want you to know, you won’t know.  I’ve heard people say that they have gay-dar, and maybe some people do, but I doubt it.  Some gay people are so comfortable about it that they will tell you straight out; and others act overtly, as a way of telling you without words.  If you like someone and are not sure about their sexuality, the best way to proceed is to be friends with them.  Good friends talk openly and trust each other.  After awhile, if nothing happens that brings you closer then friends, you still have a great friendship.  Also, you can ask questions, when you feel that it is appropriate, about meaningful relationships that they have had in the past, and see if perhaps there have been any.  Of course, some gay men have had long relationships with women to cover up, for fear of rejection, or other reasons.  If you are truly interested in a person, be a friend first.  In time, I think you will know.