Follow these simple steps: Set your multimeter to the 20V range if it’s a manual one. Just set it to Voltage if it’s an auto-ranger. Touch the negative terminal probe of the multimeter to the negative terminal of the battery. They should both be black in color. Touch the positive terminal probe of the multimeter to the positive terminal of the battery. They should both be red in color. Turn on the car headlights. Read the reading on display.