One of the paintings is the famous Mona Lisa.
After showing all four paintings, you point out, that one of the paintings is a forgery. When you show Mona Lisa a second time, she is suddenly holding a playing card in her hand. Turning the card around, the spectators are surprised to see Mona Lisa painted from behind. She has to be the fake!
But that is not all, imagine the shock when Mona Lisa reveals she is holding the same card a spectator has previously chosen!