How will you decide whether a given piece of glass is a convex lens, concave lens or a plane glass plate?

Hold the given piece of glass over some printed matter.
(i) If the letters appear magnified, the given piece is a convex lens.
(ii) If the letters appear diminished, the given piece is a concave lens.
(iii) If the letters appear to be of the same size, then it is a plane glass piece.