An evaluation method in which a capacitive sensor is placed in a region of a container, into which a granular material is fed so as to contact the sensor, and the variation in the impedance of the sensor caused by feeding in the material is evaluated. The method is particularly useful for evaluating the compactness of sand in a formwork for making metal castings. The evaluation system includes a sensor defined by plates sensitive to the variation in the dielectric; and a conditioning and control circuit for indicating the variation in the capacitance of the plates, or more generally the total impedance of the system, when the granular material is fed into the container. In one embodiment, the plates of the sensor are defined by two combs made of conducting material, having two numbers of interlacing segments, and located on a rigid or flexible support.