Without directly asking, you can try indirect cues..
For instance, ask him what he did this past weekend and once he answers (regardless of what he says), tell him that you and your friends got drunk and hit up a bunch of random bars. Once you set the stage, start naming some of these bars and make sure to name ONLY gay clubs.
If he recognizes these clubs, there might be a good chance that he is a gay. If he doesn't, he is either not gay or just trying to hide it.
Another thing you could do is ask him about his last relationship. Ask him what led to the breakup and whether "she" was problematic. Sometimes, a gay man will refer to his lover as his "partner".
Another crazy thought is to simply ask him if he is gay. If he says no and makes a big deal about you even thinking such a thing, say something that is super melodramatic, like that your brother just told you that he was gay and it has made you question the nature of all of your male friends.
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