A method of forming visible image on an on-press developable lithographic printing plate is described. The plate comprises on a substrate a photosensitive layer developable with ink and/or fountain solution and capable of hardening upon exposure to a radiation. The plate is imagewise exposed with a radiation, overall treated with a discoloring agent to form visible image or enhanced visible image, and then on-press developed with ink and/or fountain solution. The formation of visible image allows easy identification or inspection of the plate by the pressman before mounting on press.