Determination of tax evasion: notice mechanism compels payment with interest and escalating penalties to conclude proceedings. When a proper officer suspects tax shortfall, erroneous refund or wrongful input tax credit due to fraud, wilful misstatement or suppression, a show-cause notice is served requiring payment of tax with interest under section 50 and a penalty equivalent to the tax; statements for other periods may be treated as notices where grounds align. Voluntary payment before notice with interest and a reduced penalty prevents notice; payments within prescribed short periods after notice or after order with higher reduced penalties conclude proceedings. The officer must determine tax, interest and penalty by order within the statutory limitation from the annual return due date or the date of erroneous refund.