I am not sure where you can check but I would encourage you to consider another method. In certain states it is illegal, NY FL IL, but in many states it is a gray area. In NY the OAG actually charged both providers and billing services who had percentage based contracts. Being that it is the government many felt they had to pay fees because trying to fight would have been even more costly. It is just better if it is even a question to not go there. Unless you live in a state like CA where they specifically have a law that allows percentage based billing. NY has a bill on the Senate floor but it's stalled so who knows if it will ever pass.