You need to be on the ball...
... and hit em with a good come back immediately for maximum effect. Read some David DeAngelo C&F stuff (I don't think it is the end all be all of seduction as he likes to suggest but its great for situations like what you just mentioned).
"Whoa! Slow down girl, I'm not that easy."
"Oh, the old role reversal trick eh?"
"A classic case of Freudian transference there."
Or you can play it a bit tougher too (especially if you actually want the girl for a GF)
I was once called out for flirting by a girl. She said, "will you stop flirting with me!"
I apologised, stood up and went and flirted with another girl and didn't talk to her for a few weeks. She never gave me grief for being flirtatious again.
Okay, that's some tips on handling the flirting **** test on the spot. As for next time you see her...
Be careful to hide the fact you've been trying to figure out how to turn it about on her.
Now some ideas.
I would suggest playing it cool.
When it is appropriate cut off something you were about to say and say "Oh, I can't say that, you'll read into it and think I like you."
If you do play it cool its likely that she'll try to entice you to flirt with her again (even if she's not interested- although she probably is- she's a chick and will want the ego boost) say "whoa whoa, not flirting remember. I thought I told you last time!"
Let me finish by saying that flirting is like the tango... don't let a chick try to tell you that you were doing it alone.
