Apparatus for assessing field distortion includes a probe and a processor. The probe includes a mechanical fixture for placement at a location to be tested, and one or more field generators, which are attached to the mechanical fixture and are arranged to generate respective magnetic fields. The probe further includes one or more field sensors, which are attached to the mechanical fixture at known positions with respect to the one or more field sensors and are arranged to sense the magnetic fields generated by the one or more field generators and to output signals responsively to the sensed magnetic fields. The processor is arranged to process the signals so as to assess a distortion of the magnetic fields sensed by the field sensors at the tested location.