Kung Fu Maintenance shows one way how to tell hot from neutral wire when both wires are black with sensitivity adjustable non contact voltage detector.
First step was to turn off the breaker than prove off with non contact voltage detector. Separate one set of black wires, position for safest clearest test, and turn the power back on.
[On camera you can see a little residue of the faded white insulation. At the time in person all four wires looked completely black.]
OK so someone put in neutral line but they didn’t color it white or leave it white so what we are going to do is determine which wire is the hot and what we’ve done is separated one wire here but because these are so close it is picking up the signal. What I need to do is move that further away. If I had a more sensitive tester that would work better. To prove it out. So this tester is a little more sensitive. You can adjust the sensitivity on it. So I can lower the sensitivity here. As you can see we have no hot here, See no hot here, and hot, so that’s are hot and this is our neutral. Now to make it easier in the future we will wrap it with a piece of white tape and that way it designates it as the neutral. That is how you can figure out which wire is which.
[This wire run had six wires at each box, four black and two green. Each side of the conduit had three wires two black and one green. One side was bringing the power, the other receiving. Now all that was left was to repeat the procedure at the next box…]