Sterling silver is the highest commonly found grade of silver used in cutlery, decorative silver and jewellery. Silver-plated objects are made of another metal, such as nickel or copper, and coated with silver. Sterling silver is worth a lot more than silver plate, so it is important to be able to tell the difference between sterling silver and silver-plated goods, whether you are buying or selling.
Look for a mark. All American sterling (and much of the sterling from foreign countries) is marked with the number 925, or 925/1000, or the word "sterling." If the piece is American and doesn't have this mark, it is not sterling. Be wary of pieces of any origin without this mark.
Examine the piece, especially the edges, where the wear is greatest. Silver plate, especially low-quality, eventually rubs off. The base metal will shine through on worn plated items.
Nick the item discreetly with a needle, as long as you are sure you won't decrease its value. A nick in the silver will be able to show the layer of silver and the layer of plate, if the item is not sterling.
Use an acid test on the nick. Place a drop of special silver-testing solution, purchased for this purpose, on the nick. The colour of the acid solution will change. Refer to the product's packaging or instructions to determine the meaning of this colour change. This colour change will reveal if you are testing plate silver or fake silver, as opposed to real silver, but most solutions won't tell you if the item is 92.5% silver (sterling).
Learn silver marks from around the world. Foreign silver without the 925 marking usually sports a maker's mark. By reading up on trusted silver designers and companies, you can somewhat protect yourself from identifying sterling as silver plate (see resources).
You should handle the acid solution with care, as you should with all acids. Wear gloves and goggles or glasses. Avoid getting it on your skin, and wash immediately if you do spill some on yourself.