You will need to establish for the court the amount of the income that he produces. The IRS does this all the time and has written a nice audit guide on how to do this, it can be found here: http://www.irs.gov/Businesses/Small-Businesses-%26-Self-Employed/Cash-Intensive-Businesses-Audit-Techniques-Guide---Chapter-5. If you signed the joint returns showing little income while you knew the income was big then you may not be eligible for innocent spouse treatement. You may also want to amend your old tax returns to reflect the amounts that you now wish were fully reported previously. Had they been reported in the first place you could show the court the true value of the business and the income that child support should be computed from.
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You need to hire a competent family law attorney who will help you hire a competent forensic accountant. They will be able to help you establish the true level of income, despite the lack of support on the tax return.
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