"An impossible task"
I don't know about that. Difficult I'll concede, but not impossible.
There are a whole pile of questions here, even if incorrect calibration cannot be proven: When was it calibrated? When was it due for recalibration. Was it recalibrated within the manufacturer's recommended time period? Was it repaired? Were original manufacturer's certified parts used? Was it recalibrated following repairs? Who calibrated it? Were they trained? Was the operator/garda trained?
I've often wondered about these. Frankly I'd be surprised if the validation trail held up. Marathon Man - another Engineer!