California law does not require your employer to respond to inquiries regarding your work by prospective employers.
However your employer is required to provide you with an opportunity to copy your personnel file. Most employers will provide you with a copy of your personnel file upon written request.
Your employer is also required to provide you with a copy of any documents you signed while you were employed by them. Your employer must provide you with this information upon written request.
Your employer is also required to give you a copy of any background investigation conducted pursuant to credit agency laws.
If your employer wants to play hardball and not respond to inquiries regarding your employment, ask them for a copy of your personnel file. In that way you will at least have written proof of your employment.
Smart employers usually will respond to inquiries regarding ex-employees by simply providing the title of the position the employee held and the dates of employment.
There are good reasons for an employer to give this bare-bones neutral response. If the employer says anything negative regarding an ex-employee's job performance they can be sued for defamation. If the employer praises the former employee's job performance and the new employer hires them on that basis, they open themselves up to a lawsuit by the new employer should the employee shows themselves to have not been worthy of the praise.
Once again you can take action if you think a little bit outside the box. Requesting your personnel file and other documents your former employer is required to provide you may lead to a discussion with their human resources department. Perhaps they would be willing to give prospective employers the standard neutral response if they do not have to go through the trouble of copying her personnel file and finding all documents you signed for them during your employment.
Finally for a small fee, services exist that will call your former employer and claim to be a company interested in employing you. They will discover exactly what your employer is telling prospective employers.
Good luck and God bless.
Employers are not obligated to provide a response to a background check. Many employers now simply verify dates of employment because they do not want to be sued. Perhaps your former employer, if confronted, will give you a rather generic letter of recommendation setting forth your dates of employment.
If you do not have a written agreement, the former employer is not required to verify employment or provide any other information. They may if you politely request such confirmation. Good luck.