How can they confirm diagnosis between parkinsons and essential tremors?

By history & exam: Check out http://www.mayoclinic.org/diseases-conditions/essential-tremor/basics/symptoms/con-20034509. Essential tremors typically occur when attempting to do something, especially w/hands while Parkinson's disease tremors are at rest. PD also affects gait, handwriting & facial expression whereas essential tremor doesn't.

Straight forwards: Essential tremors result in shaking if arms are held upright, not at rest, Parkinson's shake at rest, not when in action. Muscle tone can increase in Parkinson's and gait also changes, but these findings not usually seen in essential tremor. L-DOPA does affect Parkinsonian tremor, but not essential usually.