Disconnect the leads to the photovoltaic cell with a screwdriver. Orient the face of the photocell toward the sky.
Set the multimeter to its 12-volt setting.
Touch the red positive lead of the meter to the red positive lead of the photocell. Touch the black negative lead of the meter to black negative lead of the photocell.
Read the meter. If the meter does not produce a reading the cell is faulty.
Open the case of the sensor with a screwdriver.
Locate the two wires attached the photocell. For battery-powered applications the cell will have a red positive wire and a black negative wire. AC units will have a white neutral wire and black hot wire.
Insert fresh batteries into a DC unit. Plug an AC unit into an outlet or turn on the switch if the unit is hardwired.
Set the Multimeter to its 12-volt setting for DC powered units. Touch the red probe of the meter to the red wire of the sensor and the black probe of the meter to the sensor's black wire. Wave your hand back and forth in front of the sensor. If the photocell sensor is working properly the switch will activate and the needle on the meter will move upward.
Turn the meter to its 120-volt setting for AC applications. Trigger the switch by moving your hand in front of the sensor. If the needle on the meter doesn't move the photocell is faulty.