Final valuation of stamps should be done by experts, since very fine details can make drastic differences in the value of a stamp. However, there are methods for consumers to use to evaluate approximate stamp value.
Confirm the issuing country of the stamp. This typically appears on the stamp, though different alphabets or writing systems may make it difficult to identify. Confirm the postage value of the stamp and, if possible, its year of issue.
Stamps that are in mint condition are worth more than stamps that have been used as postage and cancelled. A mint condition stamp may be worth 10 times the value of the same stamp cancelled. Sheets of stamps are also more valuable than individual stamps. Check the stamp for tears, creases or bending, any of which will diminish its value.
Check industry-standard price guides for stamps, such as the Scott Postage Stamp Catalog for U.S. stamps or the Stanley Gibbons Commonwealth and British Empire Stamps Catalog for U.K. stamps. Online price guides may also be helpful. If you believe you have a particularly valuable stamp, make sure it's the rare version of the stamp rather than the common version. Take valuable stamps to stamp dealers and experts for confirmation of value.