“Most people can’t fake smile,” says O’Sullivan. “The timing will be wrong, it will be held too long, or it will be blended with other things. Maybe it will be a combination of an angry face with a smile; you can tell because their lips are smaller and less full than in a sincere smile.”

These fake emotions are a good indicator that something has gone afoul.
http://www.m.webmd.com/a-to-z-guides/features/10-ways-catch-liar