There are two things to look for when a piston fails. A head or set of heads will stay in the up position until the pump is turned off and then they will drop down, and the pressure on the gauge of the water valve will be less than normal. The second is if the valve fails to rotate but the pressure on the gauge is still normal (minimum of 14 but preferred 19 to 20).
Exception: the PCC system may have one, two, or three fixed nozzles that are opposite the main drain that are not cycled by the water valve.