Here's an interesting test (via Cognitive Daily): can you tell when someone is faking a smile?
I scored 10/20 (no indication of how that compares with other people). Interestingly, I missed 9 of the first 13, then judged correctly on 6 of the last 7. Somewhere around halfway through, I decided that a sign of a fake smile was when the corners of the mouth moved up and out before the rest of the mouth moved. So I began watching exclusively for that clue and ignored any other impressions. It might have been chance, but that does turn out to be one of the cues that distinguishes a controlled smile from a spontaneous smile, and my score did improve dramatically. To see the other tells, you'll just have to take the test yourself.