I'm not sure how someone would "test" the strength of the hitch - or more properly, the strength of the bumper assembly itself.
If the bumper assembly was true custom, meaning a local welder fabbed it up for you, then he should have some idea of how tough it is, based on the metal size, the span, the weld bead size/length, the shape of the formed metal, and so forth.
If it was semi-custom, meaning designed and supplied by an aftermarket company, then they should have done some engineering (I hope) as part of the design.
One of the important points to remember is that the weight-distributing hitch (WDH) will put a substantial upward twist (torque) on the assembly. The assembly obviously must withstand this torque. This can be a tougher requirement than simple weight-bearing or weight-pulling capacity.
If, in the end, you can't reassure yourself that it is strong enough to be safe, you might consider having a commercial frame-mounted hitch installed underneath the custom bumper assembly. These are carefully engineered and speciifed for needed strength and twist resistance. Cheap insurance, relative to the potential loss of an inadequate custom job.
Bill
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