Is it true that you need to get diagnosed with gender identity disorder to get a sex change and what will they do to test it?

In general, yes. But you don't HAVE to be: most reputable plastic surgeons who do SRS/GRS require 2 letters of recommendation from 2 known and accepted gender therapists who have found that the patient cannot live a 'normal' life without the surgery. GID or Gender Identity Disorder is a diagnosis from the DSM Diagnostic and Statistical Manual of Mental Disorders and is used to describe a person who's life is so disordered by the dichotomy of being one gender but the opposite sex at the same time.

The generally accepted path of getting SRS/GRS is to have been under the care of a respected, accredited and well experienced therapist who deals almost exclusively in Gender and Sex disorders for at least 2 years. Then the patient is to live at least one year as the intended sex known as transition. Then along with another known Gender therapist or psychiatrist, the professional writes up letters of recommendation for GRS which the patient passes on to the intended plastic surgeon. Often there is a long wait for surgeries so even if the transsexual already has the cost of the surgery in hand, they may have to wait years to get the surgery.

You can't 'test' GID or 'prove' it: it is a name given to a group of mental symptoms by the medical profession for a person who is experiencing great amounts of stress due to the condition. GID is actually one of those cases of "I will know it if I see it." There have been patients who have been good liars (to their therapists and themselves) and had GRS then regretted it BUT that has only happened in less than 1% of the patients. No other surgery has that level of satisfaction.