One can determine the value of a stamp by getting an appraisal, according to the American Philatelic Society. Appraisals cost an average of $50 to $100, but cannot be used as a guarantee of the sales price of the stamp in a local market. Stamp dealers also can give a valuation of old stamps; however, the valuation is not certified, while an appraisal is.
Another common method for determining the value of old stamps is to look up the stamps in a stamp price guide, according to LoveToKnow Antiques. Use of a stamp price guide requires the owner of the stamp to determine its condition. Qualities that can affect the value of a stamp include whether the stamp is circulated, if there is any physical damage or if there are signs of repair to the stamp. This process is known as "grading the stamp." Stamp condition can be listed from the low grade of poor to the highest grade of superb. After determining the condition, the owner can look up the stamp in a variety of guides in which the value is listed under the matching condition. This is an inexact method of valuation, as the stamp owner can have difficulties determining all of the price-affecting conditions. This method is, however, a great starting point for determining whether or not a certain stamp is of value.