If there is no magnet, any magnet moved quickly across the sensor in the brake caliper should register a speed.
The sensor is a mechanical reed switch and these do fail as the internals are a bit flimsy. You can test by unplugging the sensor cable at the speedo end and using a peice of wire to short across the connector. Making and breaking the connection will mimic the reed switch and you should see a reading on the speedo.
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