I don't know if this guy has a girlfriend, should I just ask him?

Yep

Casually ask

@Mystery_Beatz what is a casual way to ask that? Because I was just going to straight up ask him "do you have a girlfriend?"

yes if course

sure why not?

@wally13 Maybe you should just be like, "hey, *his name here*, what's up?" And he'd just be like, "nothing much." And then you could be like,"wait....um......What's your girlfriend's name again?" If he says a name, he has a girlfriend, and if he says, "I don't have a girlfriend....." Then, you say in a flirty way, "then I guess I'll be your girlfriend." How's that?

I agree with @Kaitlyn except for the "well then I'll be your girlfriend part" that will make you sound desperate