Hi Sen,
In our department we have good quality weighing scales kept in calibration. We take these scales out with us and use them next to the ones we want to check, to compare readings. If any are possibly out of spec, these are taken back to the workshop for calibration. (We actually get a company called 'scaleways' in to do these calibrations for us, but you can do it yourself if you have the calibration weights)
Hope this is helpful,
Edwin.
