It is up to you to decide how you are going to prove your vehicles’ weight when they are loaded.
You could use a weighbridge, on-board scales, estimate the weight from the volume of the load, or a combination of methods.
The important thing is that you write down the method you use and explain how it works. This will be your Mass Management System.
To stay accredited you need to have documents which prove your method works and your trucks are not overloaded. In part, this means keeping your records of vehicle weights. You must also be able to prove you always follow your written procedures.