A fuel cell system including a fuel cell stack having a plurality of fuel cells, the fuel cell stack including an anode supply manifold and an anode exhaust manifold, a first valve in fluid communication with at least one of the anode supply manifold and the anode exhaust manifold, wherein the first valve includes an inlet for receiving a fluid flow and an outlet for exhausting a fluid, a sensor for measuring at least a fluid pressure at the inlet and the outlet of the first valve, wherein the sensor generates a sensor signal representing the pressure measurement, and a processor for receiving the sensor signal, analyzing the sensor signal, and determining a composition of a fluid in the fuel cell system based upon the analysis of the sensor signal.