My understanding of the National Stamping Act is that there is NO
requirement for the maker to stamp the item with I.D. information
(like 14k, 18k, Plat., Irid.-Plat., 90%Plat. 10% Irid., etc.) But if
he/she DOES stamp it, it had better be correct, with only VERY tiny
tolerance; AND it must also have a trademarked stamp. In effect saying
"I, xyz jeweler, SWEAR to the quality that I have stamped."
That said, it is pretty easy to tell platinum from all other jewelry
metals, because it is 1.7% HEAVIER than gold and about twice as heavy
as silver. If you are experienced with jewelry, you should just be
able to tell by the “heft” in your hand.
Most jewelers Rhodium plate both white gold and platinum after
polishing, so they look IDENTICAL, despite weight difference.
David Barzilay, Lord of the Rings