I have a compression jig that can measure up to 500KG - use a car jack to make the compression.

The accuracy can easily be established by using known controlled weights and measuring main scale with these - I currently have accuracies of 1g in 100Kg - not high level, like lab level scales, but OK for human scales.

The whole thing weighs about 40 KG, and when its used correctly - you can measure any digital scales that lock out after a period of time, ie I regularly test Seca 813's to their max weight of 200Kg.

I believe all scales ought to be weight check across 100% of their weight range to ensure there is no mechanical sticking or binding at some point.