In knob and tube wiring, how to identify neutral wire vs a dead wire?
Plug in an extension cord, and bring its socket end up to where you're working.  Measure voltage from extension cord "hot" to the mystery wire.  120ish volts - it's a neutral.  Significantly less - phantom voltage on a dead wire. 

To sanity check what "120ish" volts should be, measure hot v neutral on the extension cord itself.