I haven't read all the replies above, so this may have been said before. It can be a frustrating process. My psychologist who I have known weekly now for three years and I don't see eye to eye about his diagnosis of me. The psychiatrist I see for meds management (I changed recently to the new doc) has dxd me with Cyclothymic disorder and depression, which my psychologist does not fully agree with.

From this website I found the following and there is more than this on this website:
http://www.enotalone.com/article/3041.html
"At the moment, there are no tests for mental illnesses. They cannot be diagnosed by checking the blood or body fluids of the person experiencing symptoms. A diagnosis will usually be made by an experienced psychiatrist in working with other health professionals. [I can assume the psychologist a patient may be seeing is consulted.]

"At first the person will be observed for symptoms, and the doctor will use pre-determined criteria to explore the person's behaviour, then the person will be observed over a period of time.

"A full medical history should be taken and information about recent changes in the person's life should be obtained from family and/or friends. Usually one or more of these people will be involved in providing care for the relative with mental illness, and as a result will have an understanding of how the person's condition has developed.

"The psychiatrist's ongoing assessment will follow one of the internationally agreed diagnostic schedules such as the International Classification of Diseases (ICD10) or the Diagnostic and Statistical Manual (DSM4)"

This website has more information, but is similar to the above website.