You will scan and it will show a code and fault such as "00290 - ABS Wheel Speed Sensor; Rear Left (G46)"
That fault code tells you that the rear left ABS sensor is at fault, if you cleared the fault and didnt fix it, it would show the same fault. If you fixed it, cleared the fault and scanned again it would show no fault codes.
You can use a multimeter to test the resistance or the rear sensors under the seat, fix the faulty sensor and the ABS light will go off after a few miles if fixed although the fault will still be stored in the ABS ECU. I say test the rear sensors, I would put money on it being a rear one!