Something I like to do in such circumstances is ask out right but very politely:
"Susan ( place her name here), do you count your time for when you're talking to me?"
And then just look at her and say nothing. Wait for a reply.Even if it takes a while and there is an embarasing silence.
If it is true, then you probably wont see her again.
Oh, and for whatever answer she gives, just say, pensively, " I thought so".
jlp