if you want a serious answer, one I have used, but I have to be in a good mood to do so, otherwise I just BS, is
(giving her a "come on now" look)
Now is that the best you can do? That's not going to cut it.
This will 1) catch her offguard and get her smiling/aughing, if not, you need to run anyway because she's a ****
She will say 'what are you talking about"
Then you say something to the lines of "now women always talk about how bad men pickup lines are, but I would think an attractive woman like yourself has a tad bit more experience.. if you are trying to ask me out, you are going to have to come up with a better one than that, so let's try this again..come on, hit me"
never has that not worked, i mean i haven't used it alot of times, but every time I have done it, it's worked, and left the girl historical, because I end up busting her balls when she comes up with lame pickup lines.
you have to know when to shut it off because it can get stale quick. Then you take charge of the situation. Then "telling" (key word) her to give me her number.. "I got to show you how to do everything don't I". By now she's already done, you are just establishing more report.. the key isn't getting the number, the key is getting the number, and actually have her waiting on you to call her.
Then leave, when you have her at a certain high from the convo. You know you have her when she asks "when are you going to call me?"... This is when the ****iness comes out in me.. "Does it matter? you are going to answer the phone anyway, or at least you better, or else" (smiling of course). if she gives you a certain time frame to call her, adhere to it, for obvious reasons.
Bottom line, no girl that you just met asks you do you have a GF if they arne't interested in you, so you have to ASSUME the close. If not, whoose the bigger clutz? Her for thinking that is a normal question to ask someone you aren't interested in, or YOU for not taking the green light?