I purchase from reputable dealers. One is the import agent for a specific coin from a specific country. Can't get one unless you are a citizen there. But I pay for that with premiums. It's kinda like an investment for me, I'll get the premiums back when I go to sell. Otherwise, ping test or taking them to a coin store that will confirm the metals. Things like security features (watch a video about the one on England's Britannia, they're pretty interesting,) luster/patina, weight and size are all extremely difficult to reproduce with the incorrect metals. There are home electronic ways to tell what metal you have, but they can get expensive. Idk how cheap they can be had, but the x-ray ones at a pawn shop/ coin/ jewelry store should be the expensive ones, like 10+ k. Believe it or not- some fakes can be worth money, but you really have to know your stuff. Lots of history.