Although there are several signs of an insincere and false smile demonstrated here by Charlie Sheen, take note of his teeth. When a sincere smile is present, the lower teeth should not be visible. We all learned to say "cheese" when we were children - which most of the time will elicit a similar baring of the bottom teeth.
The other valuable lesson here is that even professional actors - those who make their living trying to fool us into believing they're someone they're not - cannot suppress their real feelings nearly as well as they think they can. Their true emotions will always "leak" out if you know where to look.