With that in mind, researchers from Bangor University did an experiment that discovered two things: One, there is a distinct difference between a genuine smile and a fake one, and two, your brain can spot the difference ... before you even see it.
The researchers took pairs of strangers and glued a bunch of electrical sensors all over their faces, then had the newly wired participants talk each other up. What they found is that, when a smile is genuine, we engage a completely different set of facial muscles than we do when we force our faces to contort into what the researchers called a "polite smile," or what we like to refer to as our "f**k off smile." And though these subtle muscle movements around our eyes are supposedly imperceptible without plugging our faces into complicated science machines, percept them we damn well do. When we detect that a genuine smile is imminent, we begin to return said genuine smile before the person we're interacting with even so much as manages to flash a single pearly white. So, basically, each and every one of us is a smile psychic. But only when the forthcoming smile is genuine.
Just try not to overthink it.