Look at this image of a Duchenne vs non-Duchenne smiles:

https://miro.medium.com/max/1400/0*BrX4atMJKf28fWfM

Most people perceive the smile on the left to be genuine, and the smile on the right to be fake.

But in modern Western culture, smiling is a form of politeness, and it is impolite not to smile, so people are expected to smile all the time, even when they are not really happy. So a smile should typically look like the one on the right in Western culture, if done by a regular and genuine person. A genuine person cannot do a Duchenne smile all the time, since no one is happy all the time. But a manipulative psychopath will make an effort to do a Duchenne smile all the time, since he knows this kind of smile is better at disarming people, so that he can get what he wants out of them.

So ironically, a Duchenne is the real sign of a fake, whereas the non-Duchenne "fake" smile is a sign of being genuine.