well black is not always ground, it can just as easily be hot, also there are 4 correct ways you can wire a humbucker with two coils in series in phase. and 2 more for parallel in phase. the number is reduced for a second pickup that needs to go in phase with the fist one.
so first determine if there is a sheilding conductor;
is there foil or a braid in the main sheath?
if yes check the resistance from the sheild to each of the conductors, if any conductor has a resistance reading in the single digit ohms range possibly zero the will be the ground.
if there is no continuity from the shield to any conductor then any conductor can be ground.
if there is no shielding any conductor may be the ground as far as we know.
is any part of the pickup removable, or is there any visable metal parts? i know the poles are hidden but is there maybe a brass backing plate?
if there are metal parts check for resistance from the metal part to each conductor.
if the circuit is open to all of the conductors then any conductor can be ground
next you can determine which wires go to each coil;
starting with lets say the ground wire if there is one, if there is not a specific wire that must be the ground choose one as the ground, check the resistance to each other wire, if you know the advertise dc resistance then you are looking for about half that, it can be as low as 3000 ohms as as high as 10000 ohms or more for some weird designs, only one wire should give any reading the others should show an open circuit. these two wires go to the same coil, once you have found this presumably the other two wires go to the other coil.
if the red and white wires do infact go to different coils the fact that they are twisted may be a clue that they form the series link for the correct phasing;
to test the phasing you will need to either; A) listen carefully, or B) test it.
the best way i can think of to test it would be to hook one coil to the other in series and generate a current in the wires, without sophisticated equipment you can do this by running a power tool like a battery drill in close proximity to the pickup and measure the output. set the multi meter to AC and set it to the millivolts range, the pickup to powertool placement should be precice and repeatable however you plan to do that. run the power tool and record the voltage reading, switch the wires to one coil and repeat the test, the greater voltage rating should be the the correctly phased wiring. remember the pickup and power tool should not be disturbed between switching the wires.
you can also try plugging into an amp and listening for tone and volume changes between the two possible choices.
if the instrument is assembled the combination that give more oomph is in phase, out of phase sounds thiner and loses output.
if it is not assembled and you still want to use the amp you can run a battery drill near the pickups for sound, i dont know if a difference in tonality would be heard with a sound like a drill makes so you'll have to rely on your perception of whats louder. for this placement and distace of the drill and pickup is crucial.
