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Figure out the age of the stamp. Determine the stamp's country of origin. Identify the stamp with a reference book. Determine the rarity of the stamp. Keep an eye out for error stamps.
Easier said than done! You may be able to determine the age based on clues in the design. Look for historical events or figures, or try to pick out words on the stamp. Exact years are not typically printed on stamps, so it can be quite difficult figure out exactly how old your stamp is. Head to an expert stamp dealer if you need extra help. The older the stamp is, the more valuable it will be--so it's worth the extra effort! Stamps printed within the last 70 years, even those in mint condition, probably won't be valued any higher than they were originally. As with the stamp's age, look for historical figures or events on the stamp, or words--knowing the language can help you narrow down the country. A picture of Queen Victoria, for example, is probably from 19th-early 20th century Great Britain, while a picture of the Hoover Dam may be mid-century American. Depending on your stamp, it might be easier to identify it before you try to determine the age and country of origin. After you've looked over the stamp's physical condition, you'll know enough about it to be able to find it in a reference book. American stamp collectors often use the Scott Specialized Catalogue (now available in an e-book format as well), while British philatelists tend to use the Stanley Gibbons catalogue. Check out your local library to see what options you have. You can try looking at online resources and catalogues as well, but do so with a grain of salt. They may not be as accredited or accurate as reference books. The stamp's rarity will depend on its age and quantity in its initial print run. The rarer the stamp, the higher the value; some stamp collectors even say that rarity is the single most important factor in determining stamp value, more than condition or age. Check in reference books or with a professional dealer to figure out the initial print run of your stamp. An old stamp won't necessarily be rare and valuable. The 1861 1-cent Benjamin Franklin stamps, for example, aren't very valuable because about 150 million of them were produced. Though in general you want your stamp to be as perfect as possible, error stamps are an exception. These rare stamps have an error in the design rather than the centering, perforations, etc. Error stamps are extremely valuable because of their scarcity; there may only be 50 or 100 of them in existence. Valuable stamp errors include design errors, such as a map showing the wrong border; an omission error, such as Thatcher Ferry bridge stamps in which the bridge itself was missing in the design; or an inversion, like the American Inverted Jenny stamps, which printed a biplane upside down.