About the only thing that the Contractor can sue you for is Job Payment. If Bob the Builder had WC Insurance, was properly licensed, and completed the job, you owe him the money. We are not here to help you cheat your way out of paying your contracts. Licensed Contractors are typically Independent Contractors, but you do not have to have a Contractor License to be an Independent Contractor. If Bob obeyed the law and completed the job, you do not get to control the details and game the contract; that often involves an award of Attorney Fees.
We offer general concepts, but you should give ALL your facts to a licensed Attorney in your state before you RELY upon any legal advice.
I agree with Mr. Corson. if you owe the money to the contractor, you owe the money. If he did the job, he did the job. Whether he had workers' comp insurance is irrelevant to those questions. If no one got hurt on your job, what difference does it make?
Im not really sure what is going on here, but to answer your specific question you ask for certification of workers compensation insurance. This will get you out of any payments, but the lack of wc insurance by the contractor can expose you to liability for work injuries incurred on your job