A system for detecting failures in a sensitive region of an inkjet printhead is described. The system includes an inkjet printer that uses a printhead for printing an image on a substrate. The image has a sensitive portion that is more sensitive to printing failures than the rest of the image. The printhead includes a first portion that prints an area of the substrate corresponding to the sensitive portion of the image. A radiant energy source is used with a photodetector located downstream from the printhead to detect background florescence from the substrate in the area of the substrate corresponding to the sensitive portion of the image that should be quenched by printing of the image. If background fluorescence is detected, a failure is indicated.