Is there a test for huntington's disease?

Yes. To determine whether someone has huntington's disease, a gene analysis can be done to detect whether or not a repeating-cag mutation is present on the huntington gene. If the cag sequence in the gene repeats too many times, the gene is abnormal. A meeting with a genetic counselor before and after the test is recommended to determine need for the test and to discuss the results of the test.
There is but note: Because hd follows an autosomal dominant pattern of inheritance patients who are at risk of inheriting it often look for a diagnosis. The genetic test for hd consists of a blood test which counts the numbers of cag repeats in each of the hd genes. A positive result is not considered a diagnosis, since it may be obtained decades before the symptoms begin but gives you an idea of the risk for hd.