Of course, this matter must be properly investigated & it would be ridiculous to ask for a diagnosis. nonetheless, is it true that a common first sign of parkinonism is a shaking of the smallest digit on one hand? no one has noticed any other symptom

Tremor: A tremor of the finger while it's at rest could be an early sign of Parkinson's disease. A neurologist would also look for some rigidity, slowness of motion, expressionless facial features and mild gait changes as other early signs.

Possible: I am unaware of any literature that indicates that is first sign of Parkinson. Statistically it is more likely benign tremor. Could even be simple electrolyte or magnesium deficiencu

Generalized: the tremor of Parkinson is not confined a single digit and are usually generalized to the upper extremities, head and neck

Maybe: when does the shaking occur? If it occurs at rest, it might be. If it occurs when you are using your hand, probably not. some people develop a benign tremor as they get older. It is called an essential tremor. you should discuss it with your doctor.