How do psychologist/psychiatrists *know* you're clinically depressed?

the whole "business" seems very..i dunno, what word I'm trying to think of but based on the "diagnoser's" opinion (biased?). Who is to say you're "normal" or abnormal? Maybe the patient is "suppose" to be that way? Or do the diagnoser's just try to "fix" the person so they can lead a life solely based to a degree that's functional to today's society? I feel like maybe a lot of people that gets prescribed drugs to "fix" their psychology don't really need it. But perhaps some are using it like how some people use alcohol or caffeine. I'm not saying it's a bad thing, nor am i saying it's a good thing...does anyone know where I'm getting at?