Here's the bottom line: they're likely to be found. Then you'll look bad - to your spouse, your kids, and the judge. You could even go to jail. Your tax returns can be subpoenaed. If the income you're not telling your spouse about is on there, it will be easily discovered. If it's not on your returns, that's tax evasion and is a crime for which you can be fined and imprisoned.
Be completely clear about this: lying on Family Court or Supreme Court financial disclosure forms is perjury. Perjury is punishable by fines and/or imprisonment. The Court will consider it in ordering maintenance, support or distribution of property.
Your spouse's attorney may hire a forensic accountant to go over every item of your finances. This is a financial professional who investigates and reports on your financial background for court purposes. Every check you wrote, credit card you used and cash advance you made will be looked at, and that's just for starters. It's well known that husbands and wives attempt to hide money and property in divorce situations. And forensic professionals are aware of all the tricks people use.
Be forthcoming and upfront about your finances, and you'll gain the respect of the Court as someone who is cooperative and trustworthy. The ultimate financial impact of that positive impression is likely to be more valuable to you than any cash or assets you hide, especially if your deception is discovered.
McAdams Law once represented a woman whose husband was a cab driver. He received his pay in cash and claimed poverty. They had a daughter and he was obligated to pay child support. The wife claimed he sexually abused her younger sister, and obtained an order of protection commanding him to stay away from her and the daughter. He violated the order of protection and was arrested. The firm learned that he owned two cars and had bought a restaurant, and served papers on him while he was in jail. He ultimately agreed to pay substantially more in child support than he claimed to earn on his tax returns.