Cal. Lab. Code § 3711
The director, an investigator for the Department of Insurance Fraud Bureau or its successor, or a district attorney investigator assigned to investigate workers' compensation fraud may, at any time, require an employer to furnish a written statement showing the name of his or her insurer or the manner in which the employer has complied with Section 3700. Failure of the employer for a period of 10 days to furnish the written statement is prima facie evidence that he or she has failed or neglected in respect to the matters so required. The 10-day period may not be construed to allow an uninsured employer, so found by the director, any extension of time from the application of the provisions of Section 3710.1. An insured employer who fails to respond to an inquiry respecting his or her status as to his or her workers' compensation security shall be assessed and required to pay a penalty of five hundred dollars ($500) to the director for deposit in the State Treasury to the credit of the Uninsured Employers Fund. In any prosecution under this article, the burden of proof is upon the defendant to show that he or she has secured the payment of compensation in one of the two ways set forth in Section 3700.
Ca. Lab. Code § 3711