The electrical properties of semi-insulating Cr-doped GaAs are quite sensitive to monochromatic light irradiation, and thus photoelectronic data are of great value in elucidating the nature of this material. In this paper we present a theoretical basis for the determination of the photoconductivity, photo-Hall, and photomagnetoelectric effects, measure these effects in typical GaAs:Cr crystals, and show how the results lead to useful information about substrate quality.