Bad bulbs in LED Christmas lights are usually easy to find because they'll be the only one that doesn't light up. If you have a string of lights, you can often identify the bad bulb by looking at each bulb and seeing which one doesn't light up. If you have a set of lights that are connected together, it can be more difficult to find the bad bulb. In this case, you may need to start by unplugging all of the lights and then checking each bulb to see if it's working.