The issue that the light of the ABS is turned on does not have to be of the sensors, but I explain you anyway:
the sensors are checked in two ways, first resistance is measured in the coils of each of them, for this, with a canter, you disconnect the plug of the sensor that you are going to measure, and you check the sensor. You also have to verify that the coils are isolated from mass.
Then, you put the ahead in millivolts and toured the wheel with the tips connected to the sensor, if the author puts something other than 0.00mv is that the sensor works correctly. (If you could do this last test with an oscilloscope it would be the best because you would perfectly see the wave that makes the impulse)
if all the sensors are fine, and the cables do not present cuts, you will have to check ABS or the body of motors and solenoid valves.
The latter must be looked at in a workshop by taking the system diagnosis since everything is tightly sealed.
All the best.