There are a few general rules to follow when determining the value of a stamp collection, one being that older stamps are generally worth more than modern stamps. Stamps that are bent, torn or worn thin are not as valuable as stamps in mint condition.
If the stamp still has its original gum on the back, it is worth more than stamps without it. Boxes of loose stamps are less valuable than stamps that have been preserved in books or binders. If the stamps are used and from the 1930s to present, they are usually not worth very much at all.
The U.S. Postal Service began to print more stamps to meet demands when the stamp collecting craze began in the 1930s. As a result, few stamps that have been minted since then are worth more than the price on the front of the stamp. However, the Scott Postage Stamp Catalogue can be found in most libraries, and it contains industry standard price guides for collectors to compare against their own stamps. This catalogue also contains information about which stamps are the most rare, and what the characteristics are that determine the value. Small distinctions are often the difference between a $100 and $1 million stamp.