how to determine which christmas light bulb is out

To determine which Christmas light bulb is out, you can use a light meter or a multimeter to measure the electrical resistance of the bulb. If the resistance is significantly higher than the rest of the bulbs, then that bulb is the one that is out. Alternatively, you can use a continuity tester to check the connections between the bulb and the wiring. If there is no continuity, then that bulb is the one that is out.