Inspection of a tool of a machine tool

A sensor (10) for inspecting a tool (28) of a machine tool (24) is provided that has a radio frequency transmitter (12) for generating a radio frequency signal, a radio frequency receiver (14) for generating a received signal from a received radio frequency signal, a coupling unit (16) to couple a radio frequency signal into the tool (28) and to decouple it from the tool (28), and a control and evaluation unit (20) to determine a time of flight of a radio frequency signal transmitted from the radio frequency transmitter (12) and received again by the radio frequency receiver (14) with reference to the received signal of the radio frequency receiver (14).