Most Helpful Guy
Clues are kind of worthless for a number of reasons...unless you intend to plead ignorance as a defense when his girlfriend confronts you..."Oh I didn't know he had a girlfriend."
If you really want to know the answer, you need direct confirmation. There are indirect ways to get a direct confirmation, but they all involve him going on the record, saying he does or does not have a girlfriend. Do not trust Facebook either. I've seen guys who keep their relationship status as single, even when they have a girlfriend (I'm sure there are plenty of girls who do the same).
There are many clever ways to act indirectly, but if you want him to have no idea that you're at all interested in him, you need to have some acting skills.
Say you meet a random guy. You could be like, "Hey haven't I seen you around before? I think I saw you walking by a week ago with your girlfriend."
Or: "Was that your girlfriend I saw you with last week?"
As a bonus, the guy's response may give you some clue if he's interested in you or not. If he doesn't respond, or doesn't outright deny that he has a girlfriend, then you should assume he does.
This probably only works well if you do it right when you meet a guy though, but I kind of like it a bit better than phonezoo's method, because if you just assume someone has a girlfriend without giving any reason for assuming it, it comes off a little suspicious. A girl could actually be saying that because she's not interested in the guy as more than a friend, and just wants to confirm he already has a girlfriend. Or she could be saying it because she is interested and wants to find out if he has a girlfriend. But the mistaken identity thing adds an additional layer of uncertainty. Did you really see him, or was it someone else? Did you see him with some girl that you mistook for his girlfriend? Because he's wondering these questions, he's probably less inclined to think that you're digging for information about his relationship status.1THIS IS NOT RELEVANT ANYMORE