How can a doctor determine if i had parkinsons?

Three things: Careful history, examination (with attention to lateralizing features), and in some instances medication trial. Response to medication can be a notable determinant; no specific blood test, very little that is commercially available and practical to send from a genetic standpoint.

Parkinson: Parkinson shows tremor, rigidity & hypokynesias (slow movement). The slowing is due to depletion of Dopamine content in the nervous system.