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.