It just depends on how much you are willing to spend to get what you deserve.
An attorney can get a court order that orders all banks to release his financial documents and those financial documents can be then sent to a forensic accountant. The forensic account will be able to determine #1 if they have all of the bank accounts (because people transfer back and forth at some point between an unknown account and a known accout) and #2 how much income there really was.
During my divorce my ex swore I had hidden millions, ya right, I work for a living. I had to hire a forensic account to prove exactly that, I had no hidden money. They tracked everything for three years, and I mean everything. It cost almost 10,000 dollars but considering the 1/2 of the million I was going to have to pay it was well worth it. It also helped to prove I could not afford to continue to pay him spousal support. Worth it to me. I made payments to this account for two years before I got him paid off.
We also used the same type of service to prove my ex was lying about his income, he was claiming unemployment, workmans compensation and retirement benifits all at the same time. He not only had to have all of this income considered he also had to take out a 2nd on the house to pay back what he was not intitled to. Once again, worth it to me.
Good luck