I used to calibrate weigh scales back in the day working for a testing lab and we had about 40 tons of calibrated weights and a crane truck - we would increase the calibrated weight onto a set of scales - then graph out the delta - you had to be within 1% to be considered calibrated for a scale up to 100K LBS

You could probably check your scales if you had some known weights - but it sounds like all your individual scale pads are weighing the same if you weigh the same on each scale - that's how I check them -