If you don't report all of your income, then the IRS will send you a notice in the mail. All tax forms are sent to both you and the IRS. When you file a tax return the IRS matches the forms up with what you report on your tax return. If something is missing (and a computer will always catch a W-2), then they'll send you a notice and also a bill for anything you owe. If you have a refund, they probably won't bother letting you know (they're sneaky like that).
