A device includes: a millimeter-wave radar sensor circuit configured to generate N virtual channels of sensed data, where N is an integer number greater than one; and a processor configured to: generate a 2D radar image of a surface in a field of view of the millimeter-wave radar sensor circuit based on sensed data from the N virtual channels of sensed data, where the 2D radar image includes azimuth and range information, generate a multi-dimensional data structure based on the 2D radar image using a transform function, compare the multi-dimensional data structure with a reference multi-dimensional data structure, and determine whether liquid is present in the field of view of the millimeter-wave radar sensor circuit based on comparing the multi-dimensional data structure with the reference multi-dimensional data structure.