One way of homing in on a diagnosis, whether for bodily or mental ailments, is an approach called "the empirical method."

Here's how it works: they try different drugs on you. If you respond positively to a particular medication, then it suggests that you have whatever disorder that medication is known to treat effectively.

Most pdocs will start off treating depression with antidepressants. If those meds aren't helping much, then they start giving you "mood stabilizers," which are usually anti-psychotics. Another mood stabilizing drug is lithium. If you show improvement while taking a mood stabilizer, then that suggests you may be on the bipolar spectrum.

I attended a lecture, sponsored by the Depressive Bipolar Support Alliance, where the speaker was a former research biologist who became a psychiatrist. He said that a psychiatric diagnosis is basically a more-or-less wild guess. He said people are inclined to take a diagnosis as a literal "truth," when it's really just a provisional hunch on the part of a doctor. He said a psychiatrist needs to know a patient for about 5 years before a truly insightful diagnosis can be settled upon.

Psychiatrists know very well that, much of the time, they are guessing. That's why, over the long course of treating a patient with persistent symptoms, they tend to throw all kinds of drugs up against the wall to see what sticks. Then, when a favorable response occurs, they reverse-engineer a diagnosis.