Piano keys can be thought of in two separate sections. The front wider section of the key where the fingers touch is called the 'head' and the rear more narrow section is called the 'tail'.
Ivory keys were almost always made from two seperate pieces of ivory. One piece for the head and one piece for the tail. If there is a join mark visible between the two sections, the key is most likely made from ivory. If there is no join, the key is most likely made from plastic.