A simple, easy-to-use tester indicates the magnetic alignment of coils in electric pickups. This tool is refined by Les Schatten (of Schatten Design), especially for guitar shops. It is useful for troubleshooting and testing pickups and wiring. Instructions are included. it can also be a way to determine a pickup's authenticity.

Jim Rolph explains the importance and use of the Schatten Magnet Polarity Tester.
I-5127
Jim Rolph, of J.M. Rolph Pickups in Highland Heights, Kentucky, specializes in building and
rewinding pickups. Jim introduced us to the benefits of a magnet polarity tester.
"This is the most frequently used tool in my shop, and it's something that repairmen, dealers,
collectors, players—anyone who deals with pickups—needs."
"Once a pickup leaves the original owner's hands, and it's not marked, how else are you going to
know the magnet polarity? How many times have you wired a pickup and found that it's
magnetically out of phase, even though the white and black leads looked right? You end up with
the same thin, low-powered, tinny sound as a pickup that's wired out of phase. The tester
eliminates that problem. I don't see how a shop working on electric guitars could operate without
a tool like this. There's no money in doing a job the second time!"
"When you're at a show looking to buy a guitar, knowing the polarity of the pickups can mean
the difference between buying an untouched piece or one which looks right, but has had a pickup
or two replaced. For your records, here are the usual magnet polarities on some of the most
common vintage pickups":
 Tele bridge pickups remained south-up from the start, and the neck pickups were north-up.
Later, probably in the mid 50s, the neck became south-up.
 Strats were always north-up until 1960 when they became south-up, and have remained so
except for some of their newer products such as their ‘Texas Specials.'
 The Jazzmaster came along in ‘57 with a north and a south to make them hum-cancelling
in the middle position (as did the Jaguar, Mustang, and Jazz Bass-paralleling Gibson's new
humbucking pickup, which debuted with the Les Paul that same year).
 The P-Bass has one part of the pickup north and one part south, also making it humcancelling. Early P-Basses with the Tele-style headstock had a Strat-shaped single coil
pickup.
 Gibson P-90s are always south-up, and their P.A.F. humbuckers were south-up on the
screw side, and north-up on the slug side (hidden under the covers). The later patent
number humbuckers remained the same.