|Remove photocell from
heater and connect ohmmeter on the 1000 Ohm (RX1K) scale.
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