A fluid detector for determining a presence of a fluid within a container, the detector including a sensor assembly with a lens and a sensor element that outputs a first ultrasonic signal in response to an input electrical signal. A generally cylindrical wall extends outwardly from an outer surface of the container and defines a housing with a cylindrical central bore having a base surface. The sensor assembly is coupled to the lens so that the lens focuses the first ultrasonic signal toward the wall. The sensor assembly is positioned such that it receives a second ultrasonic signal from the wall that results from the first ultrasonic signal. The sensor assembly then generates an output electrical signal corresponding to the second ultrasonic signal.