A mechanical seal monitoring system and method that measure the wear of seal faces of a mechanical seal where the mechanical seal seals a rotating machine portion from another portion of the machine. The system preferably uses a wear probe movable relative to a rotating seal component so that the wear probe can contact the rotating component. The wear of the seal is preferably determined by the relative movement that is required for the wear probe to contact the rotating component. Preferably, stress waves induced by the rubbing between the probe and the rotating component are detected by a stress-wave sensor, processed by a signal processor, and either displayed to the user numerically or reported to a monitoring computer. A signal intensity comparison method is preferably used to make the detection process insensitive to background noise generated by sources other than the contact of the wear probe and the rotating component.