What if the payor worked for cash only?
If the payor worked for cash only, it may be difficult to ascertain the exact amount of income that he or she receives. The best solution to this is to make the payor provide an accounting or provide business documents, or records, to determine what his actual source or amount of income is. Unfortunately, you will always be at a disadvantage if you are dealing with the payor who worked only work for cash. You have to rely exclusively on his tax return, since there is no W-2 provided by an employer.

