A system and method adapted to have high sensitivity to the formation of biofilm by mixed community bacteria in fluids. The system enhances corrosion imbalance by differentiating conditions between metal sensor elements immersed in the liquid being monitored. The liquid may be diverted to and flowed through a sample chamber where adjacent sensor elements may reside in different flow velocity or temperature regions. The differentiated conditions allow for different film formation on one of the sensor elements relative to the other, and also more quickly than in the main system from which the sampled liquid has been diverted. The differentiated formation allows the use of measurement of polarization current between the metal sensors to produce data with superior resolution relative to prior methods. The speed of film formation promoted by the differentiated conditions allows for determination of risk of film formation in the main system prior to that film's formation in the main system.