Ball weight refers to the weight on the front end of the trailer not carried over the axles. As a rule you should aim to have 10 percent of the total trailer weight as ball weight. This can be measured at a weigh bridge by resting the jockey wheel only on the scale.
Alternatively Hayman Reese distributors have a Ball Weight scale with which you can measure your ball weight by placing it under the coupling and winding the weight off the jockey wheel.