Look at the 1099R Form. Is he or she making overpayments to his or her retirement account? That could be a way of hiding income.
Instead of looking at the 1099R form for overpayments to his or her retirement account, consider whether your spouse is choosing in someway to overpay the IRS. Once you are divorced, he or she can file as a single individual and claim a large refund.
What if your spouse is self-employed? There are lots of ways for self employed individuals to hide income or spending.
Maybe he or she starts a separate company and the only function of that company is to receive monies that are then directed towards a girlfriend or boyfriend. What a perfect way to hide an affair! The company could even be listed at the girlfriend or boyfriend’s address.
There are other tricks as well. Whoever buys the groceries in the family could always request cash and then stash the money away in a savings account.
What are other red flags that your spouse is planning to leave you?
He or she insists on giving you an allowance and cuts you off from access to any of the financial accounts.
Your spouse may be transferring money like crazy between accounts. You will need to ask yourself what he or she is trying to hide.

At my firm, I know how important it is to pour over financial records when going through a divorce. Your spouse may feel he or she has too much to lose to open his or her books voluntarily.