You can try to file a motion to require her to do an accounting however those motions are rarely if ever granted. Contact my office for free consultation.
Accountings for child support use are very difficult to obtain. Particularly in situaitons where there's arrearages. The court will likely conclude that your ex has been using her own funds to provide for the child without your assistance so now that she's receiving it again there will not likely be any concern regarding her use of the funds.
Evan A. Nielsen is licensed to practice law in California. The information provided here is for educational purposes only and is not intended as legal advice for a particular matter. This response does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult an attorney.