You can determine a commemorative stamp's worth by looking it up in the Scott Postage Stamp Catalog. Many libraries in the United States have a copy of this catalog.
It is important to remember to check the print date of the catalog to ensure that the prices reflect the current market value. Also, it is a good idea to check on eBay to see what similar stamps are going for. The stamp's price will need to be adjusted by 20 percent to make up for the selling fees. The condition of the stamp will also determine the current value of a commemorative stamp. Mint stamps with the original gum on the back are worth more than used stamps. Stamps that have creases or tears are worth less than those that are in perfect condition. Stamps printed in the United States in the past 70 years are worth very little.
Another factor in determining worth is demand. If the stamp is rare and in high demand, the price will be higher than if there is little demand for it. This means that even if the commemorative stamp is rare, the value can still be low if no collectors are looking for it.