Maximum weight for your tow ball is specified by both the vehicle and trailer manufacturer. Typically, it is 10% of the Aggregate Trailer Mass (ATM). You must use the lesser of the two limits. For example, a dual cab ute with a maximum tow ball (TB) weight of 350kg towing a caravan with a maximum TB weight of 220kg must not exceed 220kg.
This is a key reason to purchase tow ball scales. Vehicles can also be weighed at public weigh bridges or by a mobile weighing company.