The number one thing is marks on feet. Secondly, paint.
Does it say "Hong Kong"? Is the "Pat. Pend." missing? Are the eyes a different colour than the U.S. one? Do they look airbrushed instead?
Are there no foot stamps at all?
If you can't see the stamps on the feet, you can always ask a seller. Pulling up a comparison photo of a US pony and putting them side to side should help you out, too.
If you really have no idea if its nirvana or not, you can ask us. There's no shame in asking, you never know what you'll discover!
We're still discovering variants...