How to differentiate between a genuine smile and a non-genuine smile?

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. 

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.

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.

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.

Not only are the eyes involved when a smile is genuine, but also lines around the eyes -called crow's feet- are usually formed.

Some people suggest that genuine smiles don't appear instantly, but they take their time to form. (See Why some people never smile?)

Generally, a genuine smile triggers more facial muscles than a fake smile. 

When a genuine smile is emotionally intense, the eyes might close a bit. (See Why random strangers smile at you?)