It seems like you have Child Support Services involved in your case, I suggest that you work with them by giving any and all information you have on your ex's income. You should also consider consulting with an attorney to discuss what options you have to obtain information from your ex directly via legal discovery.
Practically speaking, as already mentioned in another answer, it will be very difficult for you to prove your ex's income if he's receiving cash or using false SSN's and even more difficult to collect the support. Hang in there, be patient, and do your best to work with Child Support Services in getting the support you are entitled to.
You need to provide all the information you have too DCSS. Without concrete evidence however, it will be difficult for them to assess him at the higher income level. best of luck to you.
The Law Offices of Durham and Ng represent clients in family law matters throughout the state of California and offers free consultations. However, the above answer is NOT LEGAL ADVICE and DOES NOT CREATE AN ATTORNEY CLIENT RELATIONSHIP. The facts of each individuals case are unique and no legal advice can be offered without a thorough review of your specific facts and court filings.