To determine the coil polarity connect the coil to a voltmeter, touch one of the magnetic poles with a screwdriver and take it off quickly after a second or so. If no voltage is shown for this coil, try the other. For a short moment an either positive or negative voltage will be induced in the pickup. If it is negative, reverse the probe connections. Now note the color of the wire that is connected to the + socket of the voltmeter and find out the positive contact of the other coil in the same manner. To get the normal humbucking effect both positive leads are used as pickup leads and the other two are connected. One of the positive leads is defined as a ground lead and connected with the shielding wire.
Although this method does not make it possible to tell whether the two positive leads are the beginnings or the ends of the windings, it does none the less enable in-phase wiring if other pickups are measured in the same way. Such "tests" are absolutely safe - nothing can be destroyed by carrying them out.
Excerpt from my book Building Electric Guitars