I would ask, "What do you mean by 'seeing'?"
I try not to buy into/use the lingo like "relationship", "girlfriend", "hanging out", "not ready for a relationship." It's better to just cut through the crap and be straightforward.
If she says, "I mean seeing like dating," I would look at her seriously and say, "Would that bother you?" or something like that.
If she starts acting more loving and is like, "I want you all to myself," then this is a sweet girl, keep her.
If she says, "No, not at all, I just wanted to see where you are coming from," or any number of thousand responses along those lines, she is a b!tch.
EDIT: I am sorry, I misread your post. Would I ask that question myself? Nah- I would be looking at body language and her apparent interest in me because those things would most likely tell me if she was seeing someone else.