How to determine if a person is mentally healthy?

Judging whether someone is mentally healthy doesn't require complex theories; these four points are enough:
First, they can get along well with their emotions.
It's not that they never get angry or sad, but that they can cry when they need to, express anger when appropriate, and after losing their temper, they can gradually calm down, not bottling it up or turning their anger outward.
Second, they have a clear understanding of themselves.
They know their own strengths and weaknesses, don't constantly feel "I'm useless," and aren't inflated to think "everyone must listen to me," maintaining a reasonable self-awareness.
Third, they can handle daily troubles.
When faced with pressure from work or study, they won't give up or break down immediately; they can think of ways to get through it.
When conflicts with others occur, even if they can't resolve everything perfectly, they won't dwell on it endlessly.
Fourth, they have basic motivation for life.
They eat when hungry, sleep when tired; even if they occasionally don't feel like moving, they won't stay in bed doing nothing for a long time, and they still have some hope or interest in life.
Just like physical health, mental health doesn't require perfection; a little fluctuation that you can manage yourself is perfectly fine.