Well you can always ease into the conversation about how you assume that they have a girlfriend. It's safer to assume that he's taken rather than not. It boosts his ego (especially if he isn't taken), making you more appealing to him, and if he does happen to be taken, he won't have to feel like he has to disappoint you and make things awkward. So for example, maybe you can start talking about a cute thing this one boy did for his girlfriend and say to the guy something like "maybe you should try something like that with your girlfriend. I bet she'll love that." If he doesn't have a girlfriend, chances are that he'll be like "yeah, when I get one", "I'll remember that for when I get into a relationship", or something like that. It's not likely that he'll just go "yeah, I guess so" unless he is taken or he wasn't really listening to you. If he says "that's such a good idea. Maybe I'll do that soon", he's taken or pursuing someone.
By the way, this also works quite well with girls too, not just for guys.
