This is a pretty general question, do you have any sort of info about the horse that could help answer this question? How did you ride this horse during your test ride? What discipline do you mainly ride? How much do you want to advance in said discipline?
Many horses can be quite good at whatever they are asked to do. Sure, there are some conformations and breeds that may excel better in certain disciplines, i.e. a warmblood for dressage, quarter horse for WP, thoroughbred for jumping, but none of these are parameters that you aren't allowed to pass. Do whatever you feel comfortable and want to.
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
My horse is insanely responsive to neck reining. And she really hates riding in arena type places, she feels so free in the open fields! Maybe she was used for Western? She seemed happier when I rode her bareback then when I rode her with English tack? Thanks for your responses.