A defective roll in a strip processing line, e.g. an aluminum alloy strip cleaning line, may create an undesirable mark on the strip material. Where a plurality of rolls are in use, the defective roll is identified by providing a downstream inspection station which includes a data processor. When a mark on the strip passes the inspection station, the processor is activated whereby the rolls go through a timed opening and closing sequence. During this sequence, if no mark is observed on the strip at the time when a repeat of the mark would be expected, the lapsed time is compared to a data base and the defective roll is thereby identified.