Instructions
Go to the United States Postal Service website for the value of nondenominated stamps. Click on the link called "Quick Service Guide to Nondenominated Postage" on the USPS website.
Scroll down the page and look at the pictures of the stamps to find your nondenominated stamp.
Locate your stamp on the website. The nondenominated stamps go back to 1975, so you should be able to find your stamp's worth.
Look at the value of your stamp. It will be 1 cent or greater. If it is less than the current price of postage you will have to add another stamp to your letter to make up the difference.