If you have access to digital multimeter (or ohmmeter) you can probably set it to measure resistance with the function that a sound is made when there is no resistance.

Then measure over the pickup and earth. Turn the flywheel so that the pegs on the flywheel passes by the pickups and you should hear a beep from the meter (or se that the resistance is zero).

Please revise wiring diagram on in the workshop if you are unsure where to measure.
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)