How do people get clinically diagnosed with depression?

Is there any specific clinical test or do they just ask you questions? If it's a brain exam, what exactly do they do? If questions, what questions do they ask?
I'm considering this for my son. I'm very worried about him.

From Wikipedia

A diagnostic assessment may be conducted by a general practitioner, licensed clinical social worker, or by a psychiatrist or psychologist,[1] who will record the person's current circumstances, biographical history and current symptoms, and a family medical history to see if other family members have suffered from a mood disorder, and discuss the person's alcohol and drug use. The assessment also includes a mental state examination, which is an assessment of the person's current mood and thought content, in particular the presence of themes of hopelessness or pessimism, self-harm or suicide, and an absence of positive thoughts or plans.[1] Specialist mental health services are rare in rural areas, and thus diagnosis and management is largely left to primary care clinicians.[79] This issue is even more marked in developing countries.[80] The score on a rating scale alone is not sufficient to diagnose depression, but they provide an indication of the severity of symptoms for a time period, so a person who scores above a given cut-off point can be more thoroughly evaluated for a depressive disorder diagnosis.[81] Several rating scales are used for this purpose.[81] Screening programs have been advocated to improve detection of depression, but there is evidence that they do not improve detection rates, treatment, or outcome.[82]"

How old is your son? I would recommend a psychologist over anything else, but they can only diagnose, they cannot prescribe. There are many different kinds of anti-depressants for all different kinds and levels of depression. I have what is called dysthymia, which is considered a chronic but mild form of depression. I was put on buproprin about a year ago and the results have been fantastic.

Let me know if I can help you out in some other way
Best of Luck

A diagnosis of depression is made when a psychiatrist determines the person exhibits enough symptoms of depression for a long enough period of time. This is done through an interview, but most psychiatrists will also have patients fill out some assessment papers in the waiting room with questions pertaining to mood, energy level, appetite, anxiety, stress, etc. Usually these papers ask you to rate on a numerical scale whether you agree or disagree with the statements presented. I am providing a link to the Mayo Clinic's Depression Self-Assessment Test that you could ask your son to take. The questions on this test will be very similar to what a psychiatrist would ask. Keep in mind that special care is taken when dealing with children and adolescents, especially when prescribing antidepressants. Many antidepressants are known to increase suicidal thoughts in children and adolescents.

generally a psychiatrist evaluates the patient thru a serious of questions, and the questions vary from doctor to doctor. The doctor evaluates their behaviors too and will also talk to you about your concerns and what behaviors you have seen.