I don't know how you can determine her age yourself. If it were me, I would take her to a Vet. and they could tell you an approximate age. (I think they can tell by her teeth, but I could be wrong.)

It would be good to take her to a Vet anyway, to get a complete checkup and make sure she's healthy, and make sure she's up to date on all of her shots, and get her spayed if that hasn't been done already.

If you do take her to the Vet. and the Vet. gives you an approximate age, ask the Vet how they can tell, and then come back and post it on here. I'd be interested in knowing that, too, just for curiosity's sake.