when I was in the hospital and the psychiatrist was getting ready to prescribe lithium I had to go through blood testing in order for the psychiatrist to know how off that chemical and my dopamine levels were because low lithium and low dopamine levels is one of the ways treatment providers diagnose some forms of depression and calibrate treatment so that a person does not get over dosed on the antidepressant lithium. some forms of depression can now be diagnosed like this including hereditary depressions through the RNA gene markers. short version the tests look at the blood concentration levels then the treatment providers prescribe the correct dosage of lithium or other medications depending upon what is low in that persons blood concentration levels that is causing the depression.

this is not the only physical way in which to detect some forms of depression. some people their depression shows up in physical ways in which blood tests, MRI's, and other brain scans\tests can detect.

your treatment providers can go more into detail of what your own options are for locating\diagnosing and treating your own mental problems.