I think the only way to know would be if the supplier could tell you when you purchased it.
Along the same lines of the previous post:
In the steel business, where I primarily work, we utilize foreign produced steel all the time. We are "Made in America" because the steel undergoes a "significant" transformation here in our factory. It isn't the same size, shape, or chemical composition when it leaves the production line. So, as far as your leather goes, if you would like to say it was Made in America, you probably shouldn't buy preassembled products or packaged kits and finish them. However, if you wanted to buy a hide, (regardless of where it came from) cut it, assemble it, and finish it you should certainly be able to consider it "Made in America."