The VIN, once working, will tell you what Mercedes did to the car, not AMG. Unless the seller has documentation, verifying the AMG tuning will have to be done on your own, since there isn't much in the way of records for the pre-merger cars.

Body parts (Aero Kit) will have AMG logos and part numbers molded into them and will not be fiberglass, it should be a flexible PUR plastic.

If you're really lucky and the car was engine tuned in Affalterbach, there should be an AMG engine number stamp on the block.

There is also the possibility that AMG tuning was done by an authorized center in the US.