I don't believe it is the responsibility of the consumer to determine and pay the expense to find and correct a problem that exists with a utility that is obviously overcharging.
When a business is given a monopoly by the government, it is the responsibility of that government to insure the business is treating and charging the people who rely on that service, and have no other option, a price that is fair to both the company and the consumer.
Having the equipment and the personel that can accurately determine the correct price to charge is not the consumers responsibility.
Given a monoply it is the responsibility of the company and if the company is not willing or able to do that, then the government must intercede since the consumer has no other option.