A method and apparatus to periodically inspect and control piston movability are provided that can include using a contactless sensor system through a gas outlet port of a pressure relief device. By moving a stop, relative movement of the piston can be compared to the fixed body reflecting a movability or a blocking of the release piston. Inductive/magnetic or optical borescope sensors can detect piston movement through the gas outlet, for example. The activation function where the piston must move when activated can therefore be tested and maintained.