How can a guy tell if another guy is gay without asking?

You cant. No really, you can't. You can try based of the way they dress and talk and act but that is stereotyping someone. If a guy is gay, and comfortable with it, then he probably wouldn't have a problem telling you or anyone else, if he is not, then leave it alone. The only thing being gay means is that you are attracted to the same sex. That is it. Nothing else just comes with it. In my opinion its only appropriate to ask if you are good friends. If he says no, then either he isn't gay or doesnt wanna talk about it.