A device for detecting a light beam projecting from a lamp of a vehicle is described, and includes a plurality of photo-sensors and a plurality of indicator bulbs. The photo-sensors are arranged in a linear array along a first axis, and the indicator bulbs are arranged in a linear array along a second axis that is in parallel to the first axis. A controller is in communication with the photo-sensors and the indicator bulbs. The controller includes an instruction set that is executable to monitor signal inputs from the plurality of photo-sensors to detect presence of a light beam projecting from the lamp of a vehicle, determine a cut-off gradient line for the light beam based thereon, and illuminate one of the indicator bulbs to indicate a location of the cut-off gradient line for the light beam.