Stamp collectors dream of finding a rare and valuable stamp. However, finding and identifying a rare stamp is difficult. Identifying them requires detailed knowledge of philately (stamp collecting). Typically, stamps become rare because of errors in printing, content and perforation. Limited-issue stamps can also be rare because only small numbers of them exist. Age is also a factor. Stamps simply become more scarce as they get older. Rare stamps are becoming increasingly harder to find since collectors are continually putting them in permanent collections.
Determine if your stamp is mint or used. Mint or unused stamps in very fine condition are rarer and will have greater value. Examine your stamp closely. A mint stamp in fine or greater condition will have no defects, lack cancellation marks, and have its original gum.
Use a perforation gauge to measure the number of perforations on a stamp's edges. Stamps that look nearly identical may have different perforations, and some perforations may be rare and greatly increase the stamp's value.
Research your stamp to determine how common it is. Start with the Scott Catalogue. Further research may also be needed on a case-by-case basis. The rarest stamp in the world is the 1856 one cent British Guiana stamp (British Guiana Scott 13). Only one stamp is known to exist, and it was sold for £607,750 in 1980.
Check for printing errors. Sometimes a small batch of stamps will be mistakenly printed and distributed before the mistake is caught. A famous example is the 1918 "Inverted Jenny" airmail stamp (U.S. Scott C3a) in which a pane of 100 was accidentally printed upside down. In 2005, a plate-number block sold for £1.9 million.
Check for content errors. Compare the content of your stamp with that in the Scott Catalogue. An example is the "Legends of the West" commemorative sheet (U.S. Scott 2870) issued in 1994. Some of these stamps were mistakenly printed with the image of rodeo star Ben Pickett instead of his brother Bill. In 2009, the error sheet was worth £243 versus £21 for the non-error sheet.
Use the Scott Catalogue, the standard stamp reference guide. It is updated annually and lists every known stamp in the world (with some exceptions). Once you know your stamp's condition, the perforation count and its content, the catalogue will help you determine if it is rare or valuable.
Use tongs to handle stamps. Oils and dirt from fingers can damage stamps and greatly lower their values. A magnifying glass is a necessity to spot small differences among nearly identical-looking stamps.
Do not use a hinge to mount a valuable stamp. Using a hinge will decrease a stamp's value.