....I've recently come across some very easy methods for determining both the magnetic polarity and the electrical polarity of a pickup (both single coils & humbuckers), whether uninstalled or already wired into a guitar. These properties are important since they determine how a pickup will interact with other pickups in a guitar, i.e. if they will be phase-compatible and/or (for separate single coils connected in series or parallel) hum-cancelling. It's nice to know those things BEFORE you install a new pickup in a guitar--particularly if it's a different brand than the pickups currently in the instrument--rather than find out you have phase and/or hum problems after the guitar is fully re-assembled and played for the first time. Such problems are a real PITA when dealing with Strats or similarly constructed guitars where the strings have to be removed and the entire pickguard assembly dismounted to get at the pickup mounting and wiring. Use these techniques so that you install the right pickup the right way the first time.

Rather than try to document it all here, if the need arises, shoot me a PM and I can pass along the info and some related links.