While the propeller-head in me would want to do this in free-fall / microgravity conditions, the simplest way to do it could be to place the mirror on its back, on an extremely sensitive analytical balance before, during, and after the light is " on, "-and place the laser light source directly above the mirror, to measure how much MORE downward acceleration it experiences when a light shines on it than when it's just sitting there in the dark.
