An optical source illuminates a portion of an animal nail, and an optical sensor receives light from the illuminated portion. A processor generates a signal level, and differentiates between a "quick" signal level range (arising from illuminated living tissue in the nail) and a "nail" signal level range (arising from illuminating a nail portion with no living tissue). An indicator informs a user when the signal level is in the quick or nail range, determining the location of living tissue within the nail. Source(s) and sensor(s) may be mounted on an animal nail clipper, with illumination by the source and collection of light for receiving by the sensor occurring at a position relative to a blade so that when the signal level is within the nail range and the clipper is actuated to cut the nail, cutting of the living-tissue-containing portion of the animal nail is avoided.