It is rare to find sterling silver pieces that are not marked. If a piece is unmarked it will rarely be sterling silver. Sterling can be marked with the words Sterling, Sterling Silver, 925, 9.25 or S/S. Items like tea sets, flatware and other items are normally marked on the underside. Pure silver is too soft for jewelry and many other uses. For strength it is combined with copper or other metals to form a stronger alloy. The ratio of copper is 7.75 parts per 100. That is why sterling silver is refered to as 9.25
Check the silver marks and hallmarks. If the piece does not have an authenticating mark or is not clearly marked sterling or 9.25 or with similar known marks, it most likely is not silver. It is rare to have any unmarked pieces unless they are very old. U.S. and European silver pieces several hundred years old are usually at least hallmarked with a makers mark or guild crest. Pieces marked silverplate are another metal, usually steel, with a electroplated layer of silver. The value difference between sterling and silverplate is vast.
Use a magnet to test for fake sterling or silverplate. Silver is non-magnetic. Sterling is silver and 7.75 percent other metals, primarily copper. Silver and copper will not attract a magnet. If a magnet is attracted to the piece you are testing, it is not sterling silver.
Take a light-colored soft cloth and rub the item. Real silver oxidizes when exposed to oxygen and will leave a black mark on the cloth if the silver piece has not been recently cleaned and polished. If there is a black tarnish mark left on the cloth, the item is likely to be real silver. Silverplate will mark a cloth as well. You should test it with a magnet also to make sure there is not a silverplated base metal underneath.
Become familiar with the look of true sterling silver pieces. Examine both sterling and silverplate tea sets. Sterling will tarnish and over time have a darker blueish or blackish patina. It will not be bright and shiny if left unpolished. Areas of wear with a base metal showing through or any area of flaking will mean an item is silverplate, not sterling silver.
Place a drop of nitric acid on the piece in an inconspicuous place. Items with a large amount of copper that are silverplated will turn green like oxidized copper. Sterling silver turns a creamy light color.
Use a chemical test with a liquid known as silver reagent. Put a small drop on a inconspicuous bend or seam in the metal. If the place turns red, it is silver. Coin silver (800, or 80 percent silver) will turn brown, while 500 silver has a high copper content and will turn green.
Nitric acid and silver reagent are toxic chemicals. Handle them with gloves and take care to use them as directed on the container. If you are only in doubt about one piece, a good local jewler will test your piece for free or a small fee in many cases.
Some non-sterling pieces are marked German silver or nickel silver. These names are often used with jewelry. They are made with metal alloys to have the look of silver but often contain little or no real silver.