At this point, diagnosing depression and other mental illnesses is not a cut and dried, simple matter. There are no definitive medical tests that can be performed, and there are many types of depression that are different from each other in fairly subtle ways.
A good place to start is Depression Symptoms and Screening, which contains links to several online tests. Although these self-tests don’t take the place of an evaluation by a medical professional, they will give you an idea of what your symptoms may be saying.
Your partner in finding out whether you are depressed or not is your doctor. After ruling out any physical causes, such as thyroid dysfunction, the doctor will ask a series of questions covering family history, past and current medical problems, and current state of mind. The doctor will also try to determine if there have been past episodes of depression.
The doctor will compare your symptoms to the diagnostic criteria in the DSM-V (Diagnostic and Statistical Manual of Mental Disorders) to determine if you do have depression, and if so, which form. A good mental health professional will not rely solely on the DSM-V, however. If a patient is displaying four instead of five of the criteria, that does not mean that he or she does not need treatment. Depressive disorders occur along a continuum from mild to severe, and it’s possible to fall somewhere between them.