Are you keen on her?
If you're not, then just fob her off with something like "I'm just not interested right now."
If you are and you want to test the waters. When she asks you why you don't have a gf, you can come back with something like "Well, there's this girl that I'm actually really keen on but I don't know if she's interested though... got any tips on how I can tell?"
I know, this is AFC, but this time it is deliberate. When you deliver this one, watch her carefully. What you're looking for is a relatively strong reaction where she looks really disappointed, or really lights up with interest. You'll have to go with your gut after that.