Exciter Field Terminal Polarity

Just hooking a voltage tester between the two wires and cranking the generator should identify pos and neg...If hooked correctly will register voltage if not then switch..

I am quite unsure about the wire condition which has come out, try adding a diode in any one of the terminals in forwards bias, only one combination should work.

It shouldn't matter if the flywheel diode is in the exciter controller. If this or any other polarity dependent protection is integral to the exciter field assembly then just buzz it out with an ohm meter. You want the diode to be normally reverse biased ie the correct polarity is not a short circuit across the fields.You want to see series resistance. You can work out for yourself which way the current flows on your ohm meter.