Any large company performing analyses for a living should do it.
The best way to find honesty is to look where honesty is an advantage to the person required to be honest.
If All is going to sell gold to Bob, and Bob as well as Charlie perform analyses, it may be advantageous to All to have Charlie do an assay.
Bob will gain from assaying low (he will probably assay before buying anyway), Charlie has nothing to gain from not being honest and can be expected to assay on the mark.
All's problem is now: Does he trust Bob, and if not; is it likely that Bob will try to cheat him for more than the extra assay at "Honest Charlie's Assaying House" is going to cost?
Edit: And All might corner Bob by drilling a hole in the bar to make it look like it was sampled, thus forcing Bob to be honest without having to pay Charlie.
8)
A little theoretical, but welcome to the Forum!