I got 15 out of 20 correct on recognizing the difference between real smiles and fake smiles. Before I got the score, I figured I had done much worse. I took one of these sorts of tests in an introductory psychology class several years ago. I did slightly better than 75% then. Interestingly the facial part I used most (or was most aware of using) to tell if the smiles were real was not one they mention when they discuss the differences.
BBC Science & Nature- Homan Body and Mind- Spot the Fake Smile