This is a Japanese proverb, sometimes we see a smile from another person. Be careful smile has two aspect if the smile comes out from the inner heart that is a true smile, but when come out just make us pleasure so be careful there is something with this smile. It is difficult to distinguish a true smile and fake smile, so be careful with fake smile.
It's all in the eyes, I call them smiling eyes. It's easier to see in older people, they have wrinkles in the corner of the eye. The eyes need to work with the mouth. 😆
indeed you are right :)