Did a tour of Hastings Deering in Brisbane a couple of years ago. The people servicing the Caterpillar dump trucks on the mines. The mechanics on the mine sites sent a sample of all fluids from every oil change from the trucks to be tested for anomalies. The lab technicians could look at the filings under the microscope. Check the records on when various parts were changed last time & tell you which parts were fine & which bearings needed replacing.
Source: SHIPS SHAFTED | Daily Telegraph
And they would notice a link to dud mechanics doing the job.