You can usually tell the difference between a real smile and fake one by whether the eyes are also crinkling. A fake smile is from the nose down, a real smile is all over the face. Of course, Botox will do that too....LOL.
One thing that delighted me in India: People in India tend to walk along the sidewalk with their faces straight ahead, eyes slightly down. It's such a crowded place and I think they tend to become somewhat reserved and introspective because of that. However, catch someone's eye and give them a smile and a friendly "hello" and watch what happens. The widest smiles come out and they will greet you and quite often want to talk for a few minutes. Such lovely smiles.
Two of my little 'uns have smiles that would make your heart explode. They have 1000-watt smiles that light up a room. It's impossible to be down when you see a smile like that.
Me, I have what's called "Resting B*tch Face". I look like I'm slightly frowning when my face is at rest. And as my face gets older and I get more "jowly", it pulls the corners of my mouth down even further. I think that's what gives old folks the reputation for being crabby. They're not crabby, their faces are! Hmmmmph!