+1

If I were in your position I wouldn't ask if someone is her boyfriend. I would notice whichever guy she seems closest too, and I would assume it's her boyfriend. When he does something that seems even remotely boyfriendish, say something like, you two are lucky to be together.

In my head, by assuming she has a boyfriend, and acknowledging your assumption in the conversation like it's no big deal and isn't an issue at all, it'll through off the idea she has of your intentions.

Though, this is really just a basic trick in any social interaction. Painting a picture of yourself by acknowledging your thought process through unspoken assumptions. By doing this, you don't tell them directly what you're thinking, but you make them infer what you're thinking, or what you want them to think. They'll trust their own intuition before they trust something you tell them