Postage stamp collecting is a venerable hobby, dating back to the mid-19th century. The variety of stamps that have been produced worldwide since then is immense. Fortunately, there are many resources available, both at bookstores and online, to help you determine how much old postage stamps are worth.
The first postage stamps were produced in Great Britain in 1840, and other countries soon followed. The United States began issuing them in 1847. Early stamps were engraved, limited to one color and much smaller than today's stamps. It was not until well into the 20th century that full-color stamps began to appear.
Country of Origin
The first step in finding out how much old postage stamps are worth is identifying the country of origin. Most times, the country's name will be right on the stamp, though it may not be evident to you if it is written in another language. The denomination -- cents, pence, centimes and so forth -- is another good identifier as to where it was issued.
Once you have found the country of origin, try to pinpoint the time it was issued. If a date isn't printed on the stamp, there are several ways to deduce it. First, look at the postage rate; lower costs can signal an older stamp. Look at the image to see if it's engraved or full-color. If it's a commemorative stamp, look to see what event or accomplishment it's honoring. If those don't help, do an online search by image or subject matter -- but be sure to always include the face value in your search.
Once you have identified a stamp, it is easy enough to look it up in a reference guide. If the stamp is American, for example, check books such as the "Scott Specialized Catalogue of United States Stamps and Covers" or the "The Official Blackbook Price Guide to United States Postage Stamps." Both list all U.S. stamps produced by date, with accompanying pictures and descriptions.
Once you have found the correct listing, check to see if there are different versions of the stamp. If there are, start looking more closely at the stamp. Check to see if it is canceled or unused, if it has perforations, or if it has a watermark. Investigate any other unique features that make it stand out. Certain details can increase a stamp's value dramatically. Sometimes, the difference between a valuable stamp and one simply worth face value is the number of perforations per centimeter.
No price guarantees
Just because a book or website lists a price for a stamp, it doesn't mean you will definitely get that price. Some will sell for more, some for less. And keep in mind that stamp dealers will pay you less for a stamp than they would charge a customer for it. They have costs to cover.