They're good at questioning deductions and expenses that you tell them about. So some of donnie's misuse of non-profit funds they might catch, though they'd probably just revise his taxes and make him pay interest and penalty rather than prosecute him for that.
But if Donnie never told them about income, that's harder for the IRS to catch unless it's something the other party would have notified the IRS of. It's hard for you and me to evade tax on salary and wages because our employers tell the IRS.
But if the income is coming from overseas, for example, the foreign source might not tell the IRS, making it easier to evade.
There are many other ways to evade taxes. Mueller or say the New York attorney general may have other evidence the IRS doesn't. They may have full financial statements from all his shell companies, testimony from business partners, lenders, etc., that may point to evasion the IRS couldn't detect.
A really simple example is barter income. I paint your house in exchange for you building me a new porch. Even though no money was exchanged, we're both supposed to pay taxes on our "imputed" income.
But how is the IRS to know about this arrangement? In practice they often don't. But mueller might be able to determine that such an arrangement exists through his more extensive and thorough (hopefully) investigation and subpoena powers. A variant here is trading something at a price that isn't fair market in order to hide some other payment. E.g., I paint your house, and in exchange I buy your car from you at deep discount. I evade income tax on my labor and sales tax on the car this way.