How do I show all of my ex’s income for child support?

My children’s dad owns a restaurant in D.C. and makes alot of money. However, he shows almost no income on his tax returns. He writes off everything, takes payments under the table, and doesn’t report all of the cash he takes in. Every time we go to a child support review hearing he gets away with paying a low amount of support because he provides only his tax returns as proof of income. I know that he does these things and earns more than he is saying because I worked at the restaurant with him when we were married. How do I prove that he is earning more than he is telling the child support office? Can I bring in an accountant or some type of financial expert to look at his business and expenses? I know the child support amount he is paying is not right.