My father-in-law has a set of scales like in the pic below.
He uses them just to weigh luggage as he and the family travel alot.
Yesterday morning I took some family members to the airport to travel back to the USA, before we left we used the scales to make sure all four cases were no more than 22kg (the limit).
At the airport check-in scales all the bags were within +/- 0.2kg of 22kg.
So I guess they are pretty accurate?