Diagnosing depression is not difficult especially for a trained psychiatrist. Usually the clinical symptoms are sufficient to establish the diagnosis but your doctor may advise you some tests to rule out ailments (like hypothyroidism, hyperparathyroidism, multiple sclerosis, etc.) that can be the cause of your depression. In addition, he may also enquire into the usage of any drugs / medication that can lead to depression.
DSM-IV criteria for diagnosing depression:
According to the Diagnostic and Statistical Manual of Mental Disorders (DSM -IV), a manual used to classify and diagnose mental disorders, depression can be diagnosed in a particular case if at least five of the following nine symptoms are present at the same time for most of the days a week: