We can identify with face because when someone is depressed his face goes down with sadness and no smiling we see when there is good moment are going on.

When we are in school and he is always laughing and something happened and has been depressed we can easily understand the person by his face and the work. If the person has no mood in work as well as in study the person can be identified that he has some problem and so be has been depressed.

Most probably the person can be identified by his parents,friends and the person who is nearly touch with him.