How do I tell if my filly is a horse or a pony?

I went to a horse sale and it was sad. These were people who didn't care about their horses at all. There was a group that came in and thier hooves grew straight out from thier feet almost a foot. They went for $5 I bought a couple Foals from other groups for $10 each. One was a black paint and the one in question is just white. She is really little there was no info about her or the other except that they were weanlings. She looked so precious and she's really soft spoken, (the black is pretty wild yet) so instead of letting her go to people who were buying dozens at a time I bid and won. The thing is she is so little about 3 ft. at the whithers the black colt is a little under 5ft. I was wondering if there was any way to tell if she is a pony or if maybe she was too young to leave her mother. (People can be cruel.) We've only had one foal born out here,a mule, and I can't for the life of me remember how tall he was when he was born. But in comparison to the black she's quite small.