Updated video that addresses some people's feedback such as not needing to check every bulb, as well as using another tool/method if preferred:    • HOW TO TEST AND FIX CHRISTMAS LIGHTS ...  

The method used in this video requires a Non-contact voltage tester (affiliate link):
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Amazon.com: https://amzn.to/3F753rZ

Time index
0:22 How the lights are wired and why some can be off with others lit
0:59 How a non contact voltage tester can help find blown bulbs
1:58 Demo using the voltage tester to find the bad bulb
5:30 Use a binary search to find the bulb faster without testing every one

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