granted, there will be a bit more hum (tons more if you wired your jack backwards), and holding it near another guitar's strings would be clumsy.
but you can use a screwdriver or something with a metal that's attracted to a magnet to tell if the correct combination of pup are active, for each switch selection. just be careful not to let it slam into the pole-pieces.
also, you can use an ohmmeter to measure the resistance at the output jack to determine if the series or parallel combination of pickups measures at a value that makes sense. you'll want to have the volume at max for this measurement.
and of course don't make the resistance measurements with the pickguard plugged in and your amp on. (that could be thunderous.)
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