Consult with a local lawyer to determine whether they can send out subpoenas to the appropriate places. Also, can you receive an analysis of how your ex is budgeting his expenses. Clearly, if he has expenses, then he must have an income. Someone is paying his bills.
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You may be able to subpoena documents from his employer and from any company to which he submitted financial documents. Consult a local attorney to discuss procedures.
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Cash income cases are always difficult. An attorney could subpoena his loan applications to see what he claimed as income. Bank records are helpful if they can prove that his claimed income could not support his current lifestyle. Another option is to use wage statistic reports from the Department of Labor for what comparable employment is paid. You really need to discuss this with an experienced family law attorney.