If a return required by this chapter is not filed, or if a return when filed is incorrect or insufficient, the commissioner shall determine the amount of tax due from any information as the commissioner may be able to obtain, and, if necessary, may estimate the tax on the basis of external indices, such as number of employees of the person concerned, rentals paid by the person, the person’s stock on hand, and other factors. The commissioner shall give notice of the determination to the person liable for the tax. If the determination of tax due relates to an incorrect or insufficient return filed by a taxpayer, notice of the determination must be given not later than three years after the last day on which the return was due or three years after the return was filed, whichever period expires later; if it is determined upon audit that the tax due was twenty-five percent or more above the amount reported on a return, notice of determination of tax due must be given not later than six years after the last day on which the return was due or six years after the return was filed, whichever is later. Notice of determination of tax due for any reporting period for which a taxpayer failed to file a return must be given not later than six years after the due date of the return but if fraudulent information is given in a return or the failure to file a return is due to the fraudulent intent or willful attempt of the taxpayer in any manner to evade the tax, the time limitation herein provided for giving notice of the determination of tax due does not apply. The determination of tax due fixes the tax finally and irrevocably unless the person against whom it is assessed, within thirty days after the giving of notice of the determination, protests the determination under rules adopted by the commissioner and under chapter 28-32.