Courtesy of French Mike you can check all the ABS sensors and magnetic reluctors in the wheel bearings using a voltmeter.
The signal voltage changes from 1.7V to 0.7V as each magnetic segment passes the sensor, so jack each wheel in turn and slowly rotate it marking the switch points on the tyre wall with chalk. You should end up with 48 chalk marks.
If any gaps are in the same place each revolution then likely a damaged reluctor, and if in random places suspect the sensor or possibly a slightly too big air gap.
Other common failure causes are cable damage to the sensors, a build up of corrosion on the backplate of the rear wheels, or corroded plug pins at the ABS modulator.
Here is a front hub being tested prior to fitting. I use an extension cable made up from some old speaker wire and ABS plug and socket which saves pinning through the ABS cables. It also doubles up to allow any sensor to be plugged in to the OSF so that the speedometer can be used as a diagnostic.