An additive manufacturing machine includes a laser light source, a beam entry window, a recoater, a plurality of light sources attached to the recoater, a photosensor, and a controller. The laser light source emits laser light to selectively melt one or more portions of a working layer of a powder bed during additive manufacturing of a part. The beam entry window is positioned between the powder bed and the laser light source. The recoater moves across the powder bed to spread the working layer. The photo sensor senses intensity of light emitted by each of the plurality of light sources through the beam entry window. The controller correlates sensed intensity of the light emitted by each of the plurality of light sources through the beam entry window to corresponding positions on the beam entry window based on locations of each of the plurality of light sources.