The Artful Dodger
Member
I usually ask along the lines of "does your boyfriend study here too?" or "what does your boyfriend work with?" etc. I don't think anyone has made up a story about having a bf when asked like that, people usually don't the reaction time to lie instinctively in that situation. Or just go for it, they'll let you know very fast that way too (but may be more prone to lying). Not that all women lie or anything.
Don't ask about her boyfriend, it's obvious it's just a veiled way to ask her out. So...just ask her out.