Sometimes the only difference between stamps that look alike is their watermark. Watermarks are letters or patterns impressed into the paper used to produce certain stamps. Modern U.S. stamps don’t have watermarks, but many older ones do, in the shape of a single line or double line U, S, or P. See illustrations below.
To see if your stamp has a watermark, place it face down in a watermark tray, and pour enough watermark fluid over it to cover completely. (Never use water.) The watermark should be visible; how well it shows varies with the stamp. You may not see a whole letter or design, but only part of one. Let your stamp dry completely before removing it from the tray. U.S. watermarks are always letters like those shown. (Many foreign stamps have watermarks in the shape of a crown or other symbol alone or in addition to letters.)