Okay, in particurly the woman with the painted-looking face, you can easily hear "I secretly hate you" in the way the mouth seems to move through some practiced, obligatory motion. It's really ugly and overly pert. To aid in detecting this sort of fake smile, ask yourself if you have a suppressed urge to brick the woman in the face. Seriously, man, women seem to be adept at saying "I think you're an idiot" while saying the opposite to the unobservant. Catching them at it is the most satisfying feeling in the world. Punting them would be more satisfying if I couldn't be prosecuted for it.
One of the least difficult is the standard "since you said so" smile, which is most exemplified in the black guy. He doesn't mean anything by it. He's just giving an obligatory smile, not really caring if you can detect it as a fake. No hidden agenda, really, just an average sort of person.
Most beloved, the "make it go away" grimace. They look like deer caught in the headlights of an oncoming truck. Seriously, man, but you gotta hand it to these guys: at least they mean well by it. They're actually trying to give a genuine smile in this case, unfortunately resulting in a grimace that any idiot can detect as fake. They just look like frightened animals.
In one case, however, one can easily mistake a real smile for a grimace. The ones who shake their heads in an attempt to show disapproval are TRYING to tell you the smile doesn't mean anything, suppressing it as quickly as possible. Odds are high you've just pitched a real stinker of a joke, and they're trying to tell you it "isn't funny" ("is unsophisticated," in aspie's terms). They tend to be a bit shy about expressing their emotions, but they're an open book to the even mildly observant. Most of them are nice people.
Unfortunately, some people know how a real smile feels on the face and can emulate it with some effort. Sometimes, you can catch the muscles twitching, but it's a high difficulty rating. It's a grimacer who went out and practiced in front of a mirror for a while. It's a genuine attempt at a smile, and it comes off pretty well. Just take it as it's meant.
Lastly, sometimes, people actually are trying to express a positive emotion, but some sub-conscious part of the brain senses the smile looks a bit lousy, so they reinforce it with an up and down motion of the head to signal approval. If you're having trouble delivering a good smile even when you mean it, you can use this motion as a helper stim.
Use these guys to practice.