If you unbolt the ABS sensor, you should be able to look down into the hole and see the ABS tone ring. If it's off to one side or another, that's a problem. The sensor needs to sit right over the ring.

If it's more or less centered in the hole, then try to carefully measure how far down it is from the mounting face of the sensor on the axle to the top of the teeth - where the sensor would sit. Then carefully measure how far the tip of the sensor extends down from the mounting surface as well. The sensor needs to be pretty close to the tone ring to work - somewhere between .5 and 1mm.

With a good multimeter, try checking the resistance. I'm not sure what the 'correct' reading for a rear ABS sensor is, but just check that it's not an open circuit, or a short circuit. if your multimeter has an A/C current voltmeter setting, you could try spinning the wheel and seeing if you get a reading. It passively generates alternating current as the splines pass the sensor tip. It's not a great test that everything is 'correct', but just a good enough test to see if it's dead or not.

And then just look at the wiring on the sensor, and on the car where it plugs in.