An apparatus for detecting a predefined fill level of a medium in a container having a lid by means of a conductive measuring system, which has at least two measuring electrodes extending into the container. A measurement current flowing between the two measuring electrodes is used to detect the reaching of the predefined fill level. The apparatus makes possible a differentiated detection of accretions on the lid of the container. According to one variant of the invention this is achieved by providing a compensation electrode, which is arranged such that the degree of fouling in the region of the lid of the container is determined on the basis of a current flowing between one of the measuring electrodes and the compensation electrode, caused by conductive accretions in the lid of the container.