If you are looking for valid documentation forget it. it's designed so you can't get it and have the ivory. If it's an antique 100 years old or more and you're talking an ivory piece like a figurine usually the item itself will tell you what you want to know. Also an appraisal will work. If it's just a piece of raw ivory there is no way I know of to prove it's age. If you have a part of a tusk often times it has numbers impressed into the tusk itself or you could have paperwork from the Govt involved.

Basically the law is designed to stop the trade period.

As far as the old law was concerned having to prove anything was never the Govt's responsibility. U.S Customs has warehouses full of ivory they confiscated over the years. If you have an ivory shipment coming into the country and you can't prove how you got it you don't get it period. The difference now is interstate commerce has been added and many provisions of the old law were simply never enforced. Supposedly now they will though I have my doubts.

Basically the old law was set up so once you got it into the country you were home free and could do anything you wanted.