There was no guessing involved or hand-wavy incantations, just a thorough assessment of my state of mind, both in the moment and across a span of time. This required me to be honest, which I strongly suspect many people aren't (due to things like fear of judgement) and this will hamper the capability of the doctor in doing their job.
To put this into perspective, while ill I was relatively high-functioning. You wouldn't guess I was ill, but you would likely find me abrasive and prone to apparent flights of fancy. What the worst symptom was, the crippling anxiety, you never would've been able to detect without a goal-driven conversation - the type of conversation I have with my doctor when we meet.
I think we're still largely in the dark on the methods of action within the brain that leads to these problems, but we have actually found useful tools in resolving these problems. This isn't to say it is perfect, I was put onto a completely incorrect drug by a non-psychiatrist based on my self-reporting.
The next step is going to be neuroscience explaining the modes of action in a failure state and how these drugs restore normal function.
To anybody out there suffering with mental disease, I'm with you.
edit: throwaway account for obvious reasons