A wealth of information can be found through the bank statement of the opposing party. Not only by what can be read, but also by that which doesn't appear.
Everyone has bills, and every family has even more bills. Think of what you have; a cell phone, a landline, a housing expense, food, cable, internet, electric bill, car payment, fuel for your car, car insurance, etc. These items have to be paid from somewhere. And the most likely source, is the bank account. When these items don't show up on a bank statement there is either another bank account, they are being paid for by someone else, or they are being paid for in cash.
If your ex has direct deposit, cash on hand is going to be limited to those amounts withdrawn. Any expenses not reflected on the statement are suspicious and should be subject to more scrutiny.
If your ex doesn't have direct deposit, you need to match paystubs with deposits. See how much cash is being kept on hand and then move onto the expenses.
If your ex is the type who can provide services for cash (Construction, yard work, babysitting, etc). Craigslist is a site they'll turn to, to earn that extra income.
But, it's not enough to know that your ex is advertising on Craigslist, you need to engage him/her to gain the evidence necessary for court. And the best way to do that requires some help.
You'll need a disinterested third party to engage your ex, as a prospective customer. Ideally, an investigator is the best option. Under no circumstances should you or those close to you attempt to handle this. The court will doubt the truthfulness of those close to you.
The statement from the investigator will be the evidence you need of your ex's secret efforts. Couple that with the analysis of the bank statements and any unexplained income, and you are nearer then ever, to showing the court that there is hidden income and that your ex has been lying to the court!
Company or Corporate Expenses
During their marriage, my client's ex-husband did side work for a company that required an invoice. When, during the divorce, the required financial disclosures didn't include any money from this company, a simple subpoena to the company, of any invoices submitted on his behalf, provided evidence of all the income he had received while married and for any work performed after the separation.
It's not always that easy, but keep in mind, while your ex is trying to hide the income, there may be a business that wants the expense. The idea is to enlist the aid of a non-interested third party that wants nothing to do with illegally hiding income.
Other areas where cash is spent
These expenses crop up on occasion. Keep these in mind, especially when you hear your ex complain about the cost in court!
1. Daycare expenses
2. Car repairs
3. School expenses (tuition, supplies, field trips, extracurriculars, etc.)
If they aren't showing up on the bank statement, you should be asking how they were paid.