How about a little PCB meant for testinging pixel problems?
The idea is to have a PCB with a neopixel on it, plus two LEDs that show if power is available and if data is being sent. It would also have some resistors and a reverse polarity protection circuit to make it not break too easily. Potentially it could have three pogo pins at the end so you could directly apply it to a pixel strip.
The idea is that a tool like this would immediately tell you if it’s a the problem is no power, no data, or that the data being sent is not right. (=black)
It could potentially be made a lot more powerful with a small OLED display which could show you the actual RGB(A) values, WS2811 timing, voltages and other stuff, but then you need a microcontroller, and I’m not sure it would be cheap to make anymore.