1 The muscles around the eye don't move
When a smile is genuine, the muscles around the eyes are engaged, and so a noticeable change happens in those muscles. A fake smile usually doesn't cause that change in those muscles.
2 The whole face smiles
When a smile is genuine, the whole face appears to be smiling. When a smile is fake, the mouth usually smiles on its own, leaving the rest of the face intact.
3 Genuine smiles usually last for some time
While it's not a rule, genuine smiles usually last for a few seconds on the face even when a person looks away. The more emotionally charged the smile is, the more likely it is to last on the face.
4 The bottom teeth in the mouth don't appear
When a person fakes a smile, his bottom teeth can usually be seen through his mouth. If a smile is genuine then the bottom teeth might not appear. It's important to note that the appearing of the lower teeth doesn't always indicate that the smile is fake.
5 Lines appear around the eyes when the smile is genuine
Not only are the eyes involved when a smile is genuine, but also lines around the eyes -called crow's feet- are usually formed.
6 Genuine smiles are late coming
Some people suggest that genuine smiles don't appear instantly, but they take their time to form. (See Why some people never smile?)
7 More facial muscles are involved in genuine smiles
Generally, a genuine smile triggers more facial muscles than a fake smile.
8 Eyes close in genuine smiles
When a genuine smile is emotionally intense, the eyes might close a bit. (See Why random strangers smile at you?)