Do you mean to detect quavering in the voice? Parkinson's is known to cause muscle tics, hands trembling, etc.
I imagine you would use a computer program to examine a digitized audio file, and look for variations in volume or pitch. These might last for a fraction of a second.
I don't think that this idea will work, I know some Parkinson's sufferers and by ear I cannot detect any noticeable change in their voice compared to others of the same age. There are to many other causes of tremors in the voice (such as old age) and I doubt that you could distinguish the different types.