You will need to engage what is termed as Discovery and note up a deposition to get all of his financial documents from work to prove your allegations.
The information is for general information purposes only. Nothing stated above should be taken as legal advice for any individual case or situation.
I just want to add that it can be difficult obtaining the information you require through discovery and consulting an experienced family law attorney may be beneficial for you. Good luck.
An attorney-client relationship is not created by providing answers to this question. The answers provided do not constitute legal advice and are provided for informational purposes.
Keep in mind that you have a burden of showing what his income is. Sometimes you can back into a figure based on what his minimum monthly living expenses are.... and how he pays for them.
Also, once arrears get over $5000, DCS has a few very unpleasant surprises in store for him.
This post is not meant to be legal advice for a specific situation. Additionally, it does not constitute an attorney client relationship. It is always recommended that you consult with the attorney of your choice before taking action which can affect your legal rights.