How can you tell the difference between Parkinson's disease and huntingtons?

Very different. These are two very different diseases that can be readily differentiated by a clinical exam and history. Parkinson's disease causes tremor, slow movements an rigidity. Hd typically leads to a movement disorder known as chorea.
Genes and jerks. Huntington's movements are present at rest and during activity, whereas parkinsonian movements tend to be at rest. Hd movements can be abrupt, jerky, and quite variable from one minute to another and involve proximal limbs. Pd movements are rapid but regular, tremors, stereotyped and mainly involving distal limbs. Genetic tests can confirm hd but none exists yet for pd.