Some stamp collectors simply keep the stamps that come in their daily mail while others actively seek out old and rare stamps to add to their collection. Those who collect older stamps can benefit from knowing exactly how much their stamps are worth, especially if they intend to sell them in the future. You can determine how much your old stamps are worth by checking for certain features on the stamps in your collection.
Search a stamp catalog for your stamp. These catalogs carry pictures of each stamp and their approximate value. Stamps are generally sorted by date of issue. Check for variations to ensure you have the appropriate stamp. The American Philatelic Society recommends "Scott Standard Postage Stamp Catalogues" for finding the value of stamps.
Check the centering on the stamp. This is one of the steps of grading a stamp. If a stamp is off center, it is not worth as much as one that has been printed correctly. Turning the stamp upside down can help your eye to see if the stamp is straight and centered.
Evaluate the overall condition of the stamp. It is more difficult to find older stamps in excellent condition because of the normal aging process. Tears, creases and fading can all decrease the value of a stamp.
Check the stamp gum. Stamp gum that covers the entire back of the stamp evenly is worth more than if there are gaps in the gum or it is too heavily applied in areas of the back. The use of stamp hinges to adhere the stamp into an album decreases the value, so be careful how you display your stamps.
Check the heaviness of the cancellation. Cancelled stamps can be worth more than those that are not cancelled. However, a light cancellation is greatly preferred over one that is heavy. If the stamp cannot be seen beneath the cancellation, the stamp carries little, if any, value.
Use a stamp perforation tool to measure the perforations. The perforations on a stamp should be in perfect condition for the stamp to be worth more. The size and spacing of the perforations vary by stamp. Check for the perforations to be uniform all around the stamp with no gaps or missing "teeth."
Check online and at local stamp auctions for the rarity of the stamps in your collection. If your stamp turns out to be rare, sometimes its rarity can take precedence over any flaws that it may have. Variations and mistakes are also sometimes worth more money than a mint-condition version of the stamp.
Check the selling price of the stamp. Stamps that were printed in a higher denomination will be more likely to be worth more than those at a lower denomination. However, keep in mind that what is now considered a small amount of money was considered a larger amount of money 100 years ago.