how to find bad led bulb in christmas lights

It's not that easy. You have to first figure out which strand it is that's causing the issue, and then isolate the bad bulb. You can do this by unplugging all the strands except the one you're working on and then plugging the strand in question into a power source. Now, take a look at the strand and try to spot any burnt-out bulbs. If you don't see any, you can try testing the bulbs one by one with a voltmeter or an LED tester.