If something is sterling silver it will almost always say ss or sterling on the piece of jewelry
If its not marked it usually is Costume Jewelry

EDIT ature Article: Understanding Silver Markings

Knowing what metal you are buying can help you save money and get true quality jewelry for your money.

Last month we showed you how to understand gold markings. This month, we explore silver markings.

The Federal Trade Commission is the U.S. consumer’s protection against fraud in jewelry appraisal, identification and grading standards. This agency works with the jewelry industry so you can understand the stamped hallmarks that guarantee the purity of metal used by jewelers.

Silver is a mystical white metal is used alone and in alloys for making jewelry. Sterling silver is 92.5 percent (925 parts) pure silver and 7.5 percent (75 parts) accompanying alloy.

Like gold, silver content and quality must be accurately signified. The Federal Trade Commission (FTC) has established a set of stamps and markings to represent silver content and you should look for them when making a silver purchase. Only jewelry that is at least 92.5 percent pure silver can be called or labeled "silver," "solid silver," "sterling silver," or "sterling."

Appropriate markings include: "925," ".925," or "92.5." Though not approved, I have also seen it marked "ss". Silver-plated items must be identified as "silver plated," "silver coated," or "plated with silver."

There is a new type of silver on the market: Fine Silver. Fine Silver is 99.9 percent (999 parts) pure silver and .1 percent (1 part) accompanying alloy. It is stamped or marked as .999 or .999FS.

Fine silver is very much an up and coming item, especially with the introduction of the metal clays such as Art Clay Silver, which have come into the market. These "clays" fire to .999 fine silver - about the same hardness as 18K cast gold.

(Thanks to Linda Michel for her information on Fine Silver)