Things You'll Need
Instructions
Check to see if your stamp has no visible defects or cancellation marks and if the stamp still has its original gum. If so, the stamp is probably unused and is therefore considered to be in mint condition. This increases the value of the stamp.
Research the type of stamp to find out how many of its kind were ever printed. The fewer stamps of its kind, the greater its rarity.
Research the details of the type of stamp you have and determine if it has any errors. For example, if the stamp you have is printed backwards as compared to the type of stamp it is, this error will increase the rarity of your stamp.
Take your stamp to an appraiser to have it professionally graded if your stamp has any of the above elements of rarity. The appraiser can name the value of your stamp and may be able to give you more information about it.