Well, that's actually relatively easy if you know what you're looking for -- real smiles are known as "Duchenne smiles" and involve contraction of eye muscles. And that could absolutely be done by AI today, relatively easily.

The issue is the emotion behind it... you can be quite happy without smiling at all, and people who pose for the camera all day long learn to give a 100% convincing Duchenne smile no matter how they're feeling that day.