The majority of servo would use potentiometer to measure the position of the servo internally.
So to measure the position of the servo we just need to tap into the voltage divider. One wire to the three largest centre pin and we are done.
Warning always check the maximum voltage of the feedback before hooking the feedback wire to your micro controller. If you are running a 6V servo, plugging the feedback wire into your micro controller will smoke it.
Most device when rated at 6V will out put 7V,