If you want to be serious but less formal than a subpoena and more formal than an over the counter verbal request, the following language will put the agency on notice that you know your rights and reserve the right to force delivery of documents that are public.
Although I do not believe that any portion of my request is exempt from disclosure, if you conclude otherwise, please provide me with a written explanation of your reason(s) for withholding any of the requested information, including any statutory or caselaw upon which you intend to rely and the name or title of the person making such a determination. Pursuant to Government Code Section 6253.1 of the Public Records Act, please let me know if any portion of this request seems unfocused or in any way unclear to you so that you can determine what I am seeking.
I will pay the government entity's duly enacted copying charges for the direct costs of any required duplication. Please contact me to let me know what are the anticipated costs.
Pursuant to Government Code Section 6253, subdivision (c), I will expect a determination from you within ten days regarding this request.