Hold the lens up to a light source (80 watt light bulb, direct sunlight, etc.)  with the lens of the photocell pointed toward the light source. The resistance of the meter should be low (less than 500 ohms).

Blocking off the light source by putting thumb over opening of photocell, the resistance on the meter should be high. Replace if a change in resistance is NOT observed during the test.

Source: Desa International