The value of an old stamp collection depends on many variables. Among them are the age of the stamps, the condition of the stamps and the overall rarity of the collection. A collection of pre-1900 U.S. stamps, for example, would be more valuable than a collection of U.S. post-World War II stamps, because they are older and were produced in fewer quantities. Unused stamps are always worth more than used, or cancelled, stamps, although the vast majority of stamps, even unused, are only worth face value simply because they were produced in such large quantities.
Look for your stamps in the appropriate Scott Catalogue. Scott publishes a six-volume catalogue that includes pictures and descriptions of stamps from every country in the world that produces stamps, as well as specialised editions for U.S. stamps only.
Check out online stamp identifiers such as The Swedish Tiger, which has pictures of U.S. stamps only, or the International Society of Worldwide Stamp Collectors' Worldwide Stamp Identifier, which helps collectors identify their stamps through an extensive guide of keywords found on stamps. The Swedish Tiger also lists values for U.S. stamps, based on New York auction house sales as well as recent sales on eBay.
Go to eBay and browse through the auction site's postage stamp section. eBay typically has thousands of stamps up for bid at any one time, and most entries have detailed descriptions and photos. For the most accurate assessment of the value of the stamps in your collection, look them up under "completed auctions." That way you'll see what similar stamps actually sold for.
Write, call, e-mail or visit a stamp dealer or appraiser near you and ask if he could help you determine the value of your stamp collection. Send a photo or scan of your stamps, if you are conducting business via e-mail or by mail. The American Stamp Dealers Association has a comprehensive online directory of stamp dealers, grouped by speciality, as well as appraisers (see Resources).
Age alone is not a good indicator of value. Some early stamps were produced in such large quantities that they are only worth a few cents. Scarcity is the key.