I am still learning a lot about psychology, but I have read you can see it more in their mood and behavior. I think some of it comes down to how well you know a person. If you see they are changing or if they are willing to open up about things, you will learn a lot.
In your everyday life, there is a good chance you may notice symptoms of a mental health problem, such as rapid mood swings or anhedonia. It is important to leave diagnoses to mental health professionals