How can I determine which is the neutral wire in an old house light box?
Plug a modern (polarity-enforced) extension cord into a nearby outlet. Set your multimeter to 200V AC (or something just above line voltage where you are). Touch the negative probe to the neutral (wide in the US) slot of your cord, and the positive probe to each of the wires in your light box. Only the hot should show voltage near line-level. (The neutral may show residual or induced voltage at a much lower level.)