They don't.

I'm not saying that to insult psychiatry or the people who work within that field, it's just a fact. Properly diagnosing any kind of mental illness is very difficult. It's a process of trial and error, it can take time and for some people quite a few experiences of being misdiagnosed before they and their psychiatrist get to the bottom of things. Psychiatry is still a field that has a lot of unknowns, there's no easy way to diagnose mental illness correctly yet.