Can you tell a fake smile from a genuine one?

A smile is the universal welcome, the writer Max Eastman once remarked. But how sure can we be that a person’s smile is genuine? The answer is the empathy test, created by psychologist Richard Wiseman, which probes our ability to appreciate the feelings of others – from their appearance. A photographer asks a subject to imagine meeting an individual they don’t like and to put on a fake smile. Later the subject sits with a real friend and as they converse, the photographer records their genuine smile.
Thus two versions of their smile are recorded.
 The question is: how easy is it to tell the difference?