I have done a "Train the Trainer" course for Manual Handling,(3days with Training 2000)
I was informed that the only way to inspect harnesses and sign them off as fit for use,apart from personel use, was to take the courses offered by each harnesses manufacturer as all the harnesses are of different design and strengths.
All new harnesses carry a user manual,with full details of use and inspection details, as required by legislation If your scaffolders cannot read and follow these instructions I have doubts that they should be wearing a harness in the first place
As aom states to be fully covered in the case of a failure of a harness. Is to return it to a specielist and obtain a "Fit for Use"cert. from them