When the employer disputes whether you are an employee for purposes of Worker's Compensation, unemployment benefits, disability, etc., it becomes a fact question. You will need to prove the employment relationship for purposess of unemployment or worker's compensation. The proof will be your testimony and any documents, including any W-4, employee manual, help wanted ads, etc. relating to the work.
I will update the practice area so you can hear from Worker's Compensation lawyers, since it sounds like you have a worker's compensation issue.
Did you get paid for the one day of work? I hope it wasn't under the table, in cash. A paycheck would certainly work in your favor of proving employment.
Sounds like this is a good case for both the Labor Commission and for the Workers' Compensation Appeals Board. You may be entitled to benefits in both forums.
In either forum, you're going to have to prove employment. Paystubs are excellent. Testimony from co-workers is excellent. A copy of your time-card would be good.
You may want to consider legal representation for this as it can get complicated and contentious. Find a good WC lawyer by using the "Find a Lawyer" tab at the top of this page or at www.caaa.org. CAAA is the association for attorneys here in CA who represent injured workers.
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If you have a denied workers' compensation claim, you need an attorney to contest the denial. Lawyer up. Where an employer knowingly lies for the purpose of denying work comp benefits, that employer is guilty of fraud. You will have to prove the employment issue first, obviously. Lawyer up. Don't delay. In the meantime, make sure you are receiving treatment for your injury.
Sounds like you'll likely have to take your issue to trial and let a judge evaluate the evidence and make a finding of fact. Before going to a trial, you want to have your ducks in a row and be able to present your evidence properly. I really think you should retain an attorney for this.
Basically, you need to find people to testify, including yourself, and documents to prove the relationship. Your time card that got stamped, that's good evidence. See if you can find other time cards like it to prove that yours is from the same company. Try to find the name and address of the people you worked with that day. I would review your options with an attorney and/or hire one to help you.