Flow in a forward direction through a check valve (202) is detected by a signaling device in parallel with the check valve and sensing the pressure differential across the check valve. The signaling device outputs an electrical signal that indicates whether or not the pressure differential across the valve is sufficient to open the valve to flow. A flow restrictor such as a capillary tube (208) is in parallel with the check valve to by-pass a small portion of flow during normal operation and to equalize pressure across the check valve inlet and outlet when the check valve is otherwise closed to flow.