There’s an illustration on p15 of the guide on how to measure this with a set of bathroom scales and a block of wood.
But then you would have to take a bathroom scale along. Just imagine the troubles I would get myself in to if my captain (to whom I am married) would demand me to step on it……
For less then a tenner you can still get one of those small 50kg brass fishing scales (the type with a spring and a hook). They store easy in your box-o-junk that we all seem to carry along. Today there are also electronic versions available too but they seem to be more expensive then the classic, spring operated types. And the batteries are always dead when you need it. But if you insist, the electronic fishing scales are available from just over a tenner. (Search for fishing supply shops on the Internet).
Maybe I am not so accurate as the previous posters, if the tow ball weight is somewhere between 25 and 50 kg its fine for me. I know that I can easily lift the hitch with one arm if its weight it is around 25 kg. If it is 50 kg I can only just manage to lift it with one arm. So, if the weight is between easy and hard to lift it is OK. For reference: A bag of cement in a DIY center is 25 kg, try to lift it and then try to lift two if you happen to be there.