Well, not trusting the coin show is the whole point.
I buy my gold to the central bank since I'm NOT going to spend so much money if I'm not sure that's pure.
Regarding silver though, I cannot know the percentage of silver. I know there are reproductions and stuff so...
In the shop I buy from, the guy has some kind of liquid that he puts on the coin. If the liquid becomes red, that's silver alegedly.