You can take the brakes off and examine the sensor rings on the back side of the discs and also the sensors on the spindle and rear axle housing. Removing the sensors can be a scary thing because of the brittle plastic. I have broken one because of this - before that, the wires popped out - after that the mounting bolt broke. I learned the hard way! AGAIN! The mounting bolts may be siezed due to rust and high temps. I would lube things up if you need to replace them and then use a socket and hammer to help tap them out the backside after removing the mounting bolts.
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