Physicians diagnose Machado-Joseph disease (MJD) by recognizing the symptoms of the disease and by taking a family history. They ask detailed questions about family members who show (or showed) symptoms of the disease, the kinds of symptoms seen in these relatives, the age(s) of disease onset, and the progression and severity of symptoms. A definitive diagnosis of MJD can be made only with a genetic test. The genetic test for MJD (SCA3) is highly accurate. Those individuals who are at risk for MJD (i.e. have an affected parent), but do not have any symptoms, can undergo pre-symptomatic testing to determine whether they carry the disease-causing allele (and thus will later develop the disease). Undergoing pre-symptomatic testing is a highly personal decision that at-risk individuals should make only after considering all the potential pros and cons. Many at-risk persons choose not to undergo this test out of concern for job discrimination and difficulty in obtaining or maintaining insurance, among other reasons. For more information on genetic testing and counseling, please consult the organizations listed in the section titled "Where can I get more information?"
This answer is based on source material from the National Institute of Neurological Disorders and Stroke.