Q: What is the major clinical difference between Parkinson's disease and tardive dyskinesia patients on examination?

Answer: Parkinson's patients have difficulty moving, whereas tardive dyskinesia patients have difficulty not moving!

Tardive dyskinesia, usually a side effect of anti psychotics is characterized by repetitive, involuntary, purposeless movements like grimacing, tongue protrusion, lip smacking, puckering, pursing of the lips,  rapid eye blinking etc.