Previous pay stubs I suppose. Depending on the employer you may have something in writing. This typically wouldn't be contract if you're an employee without a union.

Someone does some digging and figures it out, and maybe five or so years later you get a check in the mail for an amount the lawyers agreed was correct.

There is nearly always a contract and the business will submit tax paperwork with your compensation to the IRS.

Under the table workers are illegal and on their own