The present invention relates to a radar level gauge comprising a reference reflector arranged at a known reference reflector level in a tank. The radar level gauge is controllable between a filling level measuring state and a proof test state. In the proof test state, the radar level gauge is configured to determine a first verification level based on a reference reflector echo resulting from reflection of an electromagnetic transmit signal at the reference reflector; determine a second verification level based on a surface echo resulting from reflection of an electromagnetic transmit signal at the surface of the product; and provide the first verification level and the second verification level to allow determination of a proof test result based on at least the first verification level, the known reference reflector level and the second verification level.