A magnetic field strength detector includes a magnetic field sensing
element that outputs voltage as a function of magnetic field strength. In
some embodiments, the magnetic field sensing element comprises a coil,
and voltage is induced across the coil as a function of the magnetic
field strength. A magnetic field strength detector also includes an
indicator that alters as a function of the voltage output by the magnetic
field sensing element when the magnetic field strength exceeds a
threshold. The alteration is substantially irreversible and, in some
embodiments, visible. The detector may be used to, for example, measure
the strength of a magnetic field generated by a degaussing device, or to
confirm that a particular medium was exposed to a magnetic field with a
strength adequate to degauss the medium.