Take a close look at the gummed side of the stamp. Use a bright light & hold the stamp at an angle to the light. You will also need a good magnifying glass. With a bit of practice you should be able to see gum striations on a mint stamp ... of course, there won't be any such striations on a stamp that has been used. With the current penchant for not canceling many stamps that go through the postal system, it can be challenging to determine whether a stamp with dull gum is mint or used. On many of the Great Americans series, the gum striations can be either vertical or at an angle to the vertical ... depends on the type of dull gum used. Added: If you have any mint State Birds & Flowers stamps, practice on them. They were issued only with dry/dull gum.