Psychology is such and inexact science. That makes it hard for even the best psychiatrists and therapists to accurately diagnose mental illness as well as to accurately diagnose which form of mental illness an individual may have.
But here’s some of the basics when it comes to diagnosing mental illness: is the person’s behavior maladaptive and unhealthy, is the person unhappy, and is the person odd or eccentric.
Of course, their’s more to diagnosing mental illness. But that’s some of the basics.