In internal medicine, drawing blood, spinal taps, radiographic studies, and exploratory surgeries are done because internists and surgeons cannot see into the body to observe disease processes in the liver, lungs, pancreas, kidneys, etc. Unlike these internal disorders, unipolar major depression is a disease that manifests in emotions, physical symptoms, thoughts, and behaviors that can be observed directly by both doctor and patient. Your doctor can find out about your internal emotions and thoughts simply by asking you. Right now, sitting in a room and talking to a psychiatrist is the best way of detecting the presence of unipolar major depression, because we can observe the depression in action.
Although scientists are working hard on improving brain scans, they have nothing that can reliably detect unipolar major depression at this time. There are no tests for the reduced serotonin or other neurochemicals involved in depression, because the amounts contained in the gaps between your brain cells are too tiny.