It depends. I don't like to get hit on at the gym, because I dont' want to be rude, and it takes time away from my lifting, tiem I dont' have to waste.
But if the girl is always approaching you, and starting up a conversation with you (not just a hi, how you doing....that common courtesy), then thats a sign she would be a good canidate to ask out.
But if you the one always doing the approaching, and she seems to be very into her workout. Then she's probably not looking to be asked out, at least not in the gym at that time.
P.S. Girls get just as fanatical about their workouts as you boys, so don't assume that you interupting hers to "chat" she will like.