FED
How else are you going to do it?
A hover track and balance would be a good start.
If they don't do that on Robbies then the pilot should at least have done a hover check and handling check without pax since he had previously felt the vibration and would instantly know if it was still there.
The old adage of 'If you think Flight Safety is expensive, try having an accident' seems to hold here - very sadly. All for the sake of a check test flight it would appear.
Perhaps he was satisfied that the problem had been reported, the engineers had done some engineering and signed it off so therefore it must be OK...............