How do researchers differentiate between Dechenne and non-Duchenne smiles?

Answer 1

Researchers in the twentieth century have also noted that in addition to the stimulation of different muscles. Duchenne and non-Duchenne smiles differ in other ways. During Duchenne smiles, the face is generally symmetrical---as opposed to say a smirk, where only one side of the mouth lifts. Duchenne smiles are also smoother in appearance; whereas non-Duchenne smiles often start or end abruptly. And lastly, genuine smiles usually last between 0.5 and 4 seconds; non-Duchenne smiles are often either more fleeting or longer lasting, depending on the social trigger.

So clearly there are physical differences between genuine and fake smiles, but are people innately aware of them? How accurate is the average person at differentiating between a Duchenne and a non-Duchenne smile?