Instructions
Check the original price of the stamp. The United States Postal Service has a pdf file (see Resources) you can download that lists the postage rates for U.S. stamps.
Research stamp collectors' magazines and online forums. There are several sites that will allow you to search for a specific stamp's value. For example, Find Your Stamps Value (see Resources) has a free guest search service to help you identify the stamp and its value.
Check auctions. Search for the specific stamps on eBay or other auction sites and see what the going rate is. StampWants.com lists specialty stamps for auction.
Ask collectors. Talk to avid stamp collectors and ask their opinion on what the stamp might be worth. Attend a stamp show and ask fellow collectors about the worth of various stamps. There are also stamp dealers you can contact about the worth of your stamp. They typically offer professional stamp appraisal services for a fee.