Things You'll Need
Instructions
Review the condition of the stamp. Has it been canceled? Does it still have gum on the back? Is the design well-centered? Is it torn or creased? All of these aspects can affect a stamp's value.
Look at the stamp's design. Does it appear to be missing any colors throughout the design? Are there perforations all the way around the stamp? If no, do they appear to have been cut off by someone? If not, you could have an error pressing worth hundreds of dollars or more, depending on the stamp and the error.
Obtain a price guide for U.S. stamps. The U.S. Postal Service publishes an annual price guide, as does Scott Publishing Company. These guides cost between $8 and $20 as of 2010. If you don't want to invest in a price guide, you could go online to Mystic Stamp Company's online catalog and just look up 1973 stamps to find their cost. If you're insuring your collection, you can use the prices listed as "replacement cost."
Consult a recent error-stamp price guide dealing if your stamp looks out of the ordinary. Look up the "Scott Catalogue of Errors on U.S. Postage Stamps, 15th Edition." You may also want to look into books dealing with forgeries. If you think that you have a real rarity, contact a dealer or appraiser.