Whether or not someone can make you smile is what's known as their smileability. Can they make you smile? If they have good smileability they can. That's how you can tell.
It's important to know someones smileability, as when you need cheering up you can go a find someone with a high smileability rating, knowing they'll make you smile.
Note: Not to be mistaken for smileable
I know, she needs to find someone with lots of smileability, and quickly, to cheer her up and get her smiling again!
Kim's feeling much better now though, she's got a guy with maximum smileability, so they do exist!