Yesterday, George Clooney was arrested while protesting in front of the Sudanese Embassy. He was charged with the disorderly crossing of a police line. Only two days before he had dined with the President in the White House and testified before Congress regarding the mounting crisis in South Sudan.
In the above image, during a casual moment at his congressional testimony, Clooney certainly displayed all the components of a sincere smile: partially/mostly closed eyes with the necessary concave-up furrow in each lower eyelid and an open mouth with only the top teeth showing.
I want to call attention to George's forehead though. While there are some slight passive wrinkles in his forehead (Mr. Clooney is 50), there is no contraction of the actors forehead muscles. When the forehead does contract however, the tissue elevates and pulls the eyebrows up along with it (most don't realize that the eyebrows lower slightly during a sincere, joyful smile - although this is difficult to see dynamically). For a smile to reflect authentic happiness-joy, the forehead must be relaxed.
One common type of feigned smile, is the "false-surprise smile" aka the "dropped-jaw smile" (see Blake Lively example this: Secret # 133: Surprise or a Smile or Neither?) which is easy to spot because of the wide open mouth, elevated eyebrows and raised & furrowed forehead.
The take-home message: If the forehead is contracted, the smile is insincere. No matter what the rest of the face looks like - always evaluate the forehead.