In a liquid sensing system, an RF signal is applied to a series-resonant circuit. The coil of the resonant circuit is placed proximate to a fuel tank, causing electromagnetic radiation to propagate into the fuel space. The fuel acts as an electrical load to the resonant circuit in a manner proportionate to the volume of fuel in the tank and/or to variations in electrical properties of the fuel itself. The loading effect of the fuel can change the resonant frequency and/or the Q of the resonant circuit. The loading effect of the fuel is determined by monitoring a change in one or more electrical parameters associated with the excited resonant circuit, such as a voltage across the resistor in the resonant circuit. Changes in this voltage are analyzed by a controller, the result of which is used to output a value indicative of level and/or composition of the fuel.