how to know which christmas light is bad

It's hard to tell which Christmas light is bad just by looking at it, but there are some things you can do to check. First, make sure the bulb is screwed in tightly. Next, try plugging the light string into an outlet to see if any of the bulbs light up. If not, use a multimeter to test the voltage across each bulb. If it reads zero volts, the bulb is bad. Another way to check is to gently tap on the bulb and listen for a clicking or buzzing sound. If you hear a sound, the bulb is bad.