If the horse is safe to ride, you can test if the horse responds to neck reining- if she does, it's likely she has had some western training on her at some point. If she doesn't, odds are she was english (probably hunt seat? I see alot of hunters now a days.)
Also, if you lunge her, it is possible you might be able to tell if she's trained western/english by how she handles the speed of her gaits- this is NOT always true though. My trainer has a horse who's very much a Saddleseat horse with a rider on his back, but if you were watching him be lunged, you'd swear he was the perfect western pleasure horse.
Do you have her papers? Have any way to contact a former owner?
It's possible, if you have a show name, that she's got a record online somewhere.
However, even if the horse was trained X, you can always reteach her to do Y with you :)