A genuine smile lights up the whole face — especially the eyes — while a fake smile moves only the mouth. Robert Greene singles this out as one of the highest-value non-verbal skills to master, particularly in romantic contexts: reliably telling real warmth from performed warmth changes how you read interest, deception, and manipulation attempts.
This is a specific application of the broader discipline of Body Language Reveals What Words Conceal — "muting the words" and watching the face directly. It also functions as a first-pass filter before the harder tell of Dead Eyes as a Narcissist Tell: a practiced deceiver can fake a smile's shape but has more trouble sustaining fake eye engagement.
Apply: when someone smiles at you, watch whether their eyes visibly brighten and engage along with their mouth — not just whether the mouth curves up. Treat mouth-only smiles as a signal to slow down and gather more information before trusting the warmth as genuine.