Many metals look like they are made of silver but, in fact, do not contain any silver at all. In the marketplace, dishonest or uninformed people try to pawn off jewelry and other items as sterling, when the items are really made of base metals and are polished to resemble silver or are electroplated with sterling silver (which will eventually wear off if the piece is worn often enough). Sterling silver is 92.5 percent silver and and 7.5 percent copper, and this is why U.S. law mandates that sterling silver be stamped ".925" somewhere on the item. Since not everyone follows laws, there are a few ways to tell if what you have is truly sterling silver or not.
Examine the piece of jewelry closely using a magnifying instrument (jeweler's loupe or other magnifying glass). Look at the clasp. A tiny ".925" or "S.S." indicates that the piece is sterling silver. (A ".999" tells that the item is of fine silver--more silver content that sterling, and ".900" means it is made of coin silver--less silver content that sterling.) Unscrupulous dealers may attach a sterling clasp to a cheap base metal chain and try to convince buyers that the whole chain is sterling. Look carefully to see that the clasp and chain both appear to be the same color and have the same degree of tarnish. Also, most good sterling chains will have soldered links.
Look carefully at the pendant. It, too, should be marked with a tiny ".925" or "S.S." if it is made of sterling silver. This mark is usually on the bail of the pendant. If you see the letters "S.P.," they mean "silverplate" which is NOT sterling silver. When a base metal item is put into a solution containing silver and an electric current is run through the solution, a thin coating of silver adheres to the item, and it is called "silverplate."
If there is no ".925" or "S.S." on the piece, you may test it with nitric acid. In an inconspicuous place on the item, lightly scratch or scuff a small spot and apply a tiny drop of nitric acid to the spot. Silverplated brass, nickel silver or low quality silver alloys will turn green when a drop of nitric acid is applied because of the high copper content. Sterling will turn a creamy color. If it turns black, it is coin silver. When testing items you suspect may be silverplated, use a small file to cut through any plating or lacquer in a discreet area on the item. If you see a gold color, you can assume it is brass that has been silverplated.
Another test is to use a magnet. Sterling silver will not stick to a magnet, but just because the piece does not stick does not mean that it is sterling silver. Therefore, use this only as a secondary test.
Local jewelers and pawn shops can test items for you, too. You can also purchase testing kits from jeweler, lapidary or rockhound supply stores.
Practice testing on pieces that you do not value.
Nitric acid can burn skin and work surfaces, and can damage eyes, so use extreme caution when working with it.