I'm not sure about the teats. One of my mares is a broodmare, so she's bred all the time and they never go down. The other one had a year off, but her 18 mth old nearly grown filly kept sneaking a drink until not too long ago, so she's still fairly big too. But I think they go back down and wouldn't be a reliable way of telling for sure. Your vet could probably palpate her and tell you for sure. Are you thinking of breeding?
I haven't decided yet. After we get our own place, I might. I can't afford to stable another horse.