The invention provides a method of detection of an imprint of a postal indicium at a location on a mail piece. The method involves utilizing a sensor to scan along a band on the mail piece to detect a sequence of transitions between light and dark reflectance areas within a band extending across the said location. AN indication of the presence of the imprint of the postal indication is generated in response to detection of a transition succeeding a predetermined number of initial transitions at a start of the sequence of transitions.The invention also provides apparatus for imprinting postal indicia on mail pieces. The apparatus comprises a printer operable to print a postal indicium in a required location on the mail piece. A first sensor is provided that is responsive to reflectance transitions between light and dark along a band of the mail piece extending across the said location. The first sensor generates a sequence of first signals corresponding respectively to reflectance transitions along the said band. The apparatus further includes means operative in response to a first signal occurring after a predetermined number of said first signals at a start of said sequence to generate a second signal which is indicative of a postal indicium imprint on the mail piece.