Psychologists call a genuine smile a Duchenne Smile after Guillaume-Benjamin-Amand Duchenne de Boulogne, whose place in history is derived by his passion for shocking people in the face.
Duchenne lived in Paris and zapped people throughout the mid-1800s in an attempt to figure out whether or not the muscles of the face were connected to the soul. He defined and catalogued the eye muscles that are connected to authentic smiling.
-From "You are not so Smart" blog- Real or fake smiling. Involuntary or voluntary? “Most people are surprisingly bad at spotting fake smiles. One possible explanation for this is that it may be easier for people to get along if they don’t always know what others are really feeling.” - BBC Science and Nature Website.
The muscles which control the cheeks and the muscles which control the eyes get switched on and off by different parts of your brain. The parts of your brain which make up the conscious, in-control you are the same parts which can yank at your mouth and fake a smile. Real smiles include the muscles around the eyes, which are harder to control and thus are less likely to squinch up when you are committing grin fraud. A BBC test you can take online here to see how good you are at spotting real and fake smiles.