Assessment apparatus is provided, including a set of one or more radiators, which is adapted to generate an energy field at at least one fundamental frequency. A receiving sensor is adapted to generate a first signal responsive to the energy field when an interfering article is at a first location relative to the receiving sensor, and is adapted to generate a second signal responsive to the energy field when the interfering article is at a second location relative to the receiving sensor. A control unit is adapted to (a) receive and analyze the first and second signals, in order to compute a fingerprint signal characteristic of the interfering article, and (b) store, in a database, data indicative of the fingerprint signal, in association with an identity of the interfering article.