Whenever the Department, in examining and auditing the records of any taxpayer, or from other information, shall ascertain that the amount, or amounts, previously paid by any taxpayer for any period, is incorrect, the Department shall compute the correct amount of tax due. If it appears that the amount paid by the taxpayer is in excess of the correct amount due, such excess shall be refunded to the taxpayer under the rules and regulations of the Department. If it appears that the amount paid by such taxpayer is less than the amount due, the Department shall compute the amount of such deficiency and shall notify the taxpayer, and shall demand payment therefor. If such deficiency is not paid within thirty days from the date of such demand, the Department shall make an assessment against the taxpayer of the amount due and shall add a penalty of one-half of one percent per month from the date such taxes, or any part thereof, became due; provided, however, that if the Department be of the opinion that there was a wilful or fraudulent intent by the taxpayer to evade the tax due, it may assess a penalty of twenty-five percent of the tax.