Instructions
Determine whether the stamp you wish to value is a nondenominated stamp. If you possess a nondenominated stamp, there are several online resources you can use to help you establish its value. Consult the Postal Service's website itself, where you can reference Quick Service Guide 604a at http://pe.usps.gov/text/qsg300/Q604a.htm. You'll find that this guide displays images of all current nondenominated stamps along with a description, the date issued, and the value of each stamp.
Another resource you may wish to use is the Knottywood Treasures site, which offers more examples of regular issue nondenominated stamps that may be less-than-current. You can access this resource at http://www.knottywood-treasures.com/id39.html.
Decide if you wish to value your stamp for collecting purposes. If you're trying to value a stamp for this reason, consult a postage stamp valuation catalog. The Scott Standard Postage Stamp Catalogue is considered the "gold standard" of stamp valuation, and the Stanley Gibbons Stamp Catalogue is also well-respected in the professional collecting community. You can find either of these resources in bookstores or the library.
Consult online resources. Industry websites can help you value your current U.S. postal stamp as well. You'll find that FindYourStampValue.com offers a searchable database and a wealth of information on older stamps as well as some newer stamps. You may also wish to explore StampCatalogue.org, which offers a listing of stamps organized by year in order to help you quickly locate and value your stamp.
Consider consulting with a professional regarding your current U.S. postal stamp. The American Philatelic Society at www.stamps.org offers resources to help you find local dealers, clubs, societies and shows and exhibitions.