If you want to sell precious metal, there are many ways to tell if your gold or silver items are plated or solid. Most gold, silver, and platinum pieces have hallmarks stamped on them telling the consumer what alloy the metal is.
The most common solid gold hallmarks are the art stamps; 8k, 9k, 10k, 14k, 18k, 21k, 22k. Other hallmark stamps include; 333, 375, 417, 583, 585, 750, 875, 916, 917, 999. These all represent the permillage gold in the alloy of the item, 333 being 33.3% or 8/24ths gold (8k), 417 being 41.7% or 10/24ths gold (10k) etc.
Beautiful genuine 9k hallmark.
Common silver hallmarks include; fine silver, ster, stg, 800, 830, 925, 950, and 999. Common platinum hallmarks are platinum, plat, platine, Pt, 850, 900, 950, and 999.
Plated items may have the following stamps or markings on them: HGE (hot gold electro-plate), RGP (rolled gold plate), 1/10, 1/20, 1/40, GF (gold filled), GP (gold plate), GE (gold electro-plate), EPNS (electro-plated nickel silver), A1, Sheffield, silver over copper, and silver over brass etc.
Many plated items are plated over nickel or iron. These items will be magnetic. Gold is not magnetic, so if you use an ordinary fridge magnet, you can test to see if an item is solid gold (non-magnetic) or plated (magnetic). However, items plated over brass will not be magnetic and are harder to identify.
In most countries, national law requires gold jewelry to be marked with its art or fineness. However, the art mark is no guarantee of gold content. Canada, for example, has laws that require jewelry to be marked with both the art and the makers mark. Marking of jewelry is usually done by the manufacturer without any independent check. Thus your caratage conformity is not guaranteed and you rely on the jewelers integrity. Unfortunately, undercarating of jewelry is quite common in some countries.
Anyone can purchase a gold or silver art or fineness stamp on the internet and stamp substandard, counterfeit, or worthless items.