Section 35 ILCS 5/904 - Deficiencies and Overpayments(a) Examination of return. As soon as practicable after a return is filed, the Department shall examine it to determine the correct amount of tax. If the Department finds that the amount of tax shown on the return is less than the correct amount, it shall issue a notice of deficiency to the taxpayer which shall set forth the amount of tax and penalties proposed to be assessed. If the Department finds that the tax paid is more than the correct amount, it shall credit or refund the overpayment as provided by Section 909. The findings of the Department under this subsection shall be prima facie correct and shall be prima facie evidence of the correctness of the amount of tax and penalties due.(b) No return filed. If the taxpayer fails to file a tax return, the Department shall determine the amount of tax due according to its best judgment and information, which amount so fixed by the Department shall be prima facie correct and shall be prima facie evidence of the correctness of the amount of tax due. The Department shall issue a notice of deficiency to the taxpayer which shall set forth the amount of tax and penalties proposed to be assessed.(c) Notice of deficiency. A notice of deficiency issued under this Act shall set forth the adjustments giving rise to the proposed assessment and the reasons therefor. In the case of a joint return, the notice of deficiency may be a single joint notice except that if the Department is notified by either spouse that separate residences have been established, it shall issue joint notices to each spouse.(d) Assessment when no protest. Upon the expiration of 60 days after the date on which it was issued (150 days if the taxpayer is outside the United States), a notice of deficiency shall constitute an assessment of the amount of tax and penalties specified therein, except only for such amounts as to which the taxpayer shall have filed a protest with the Department, as provided in Section 908.