I have heard that a m'zuza must be checked twice every seven years by a qualified sofer. I have also heard that, assuming it's known to have been kosher at one point, it need only be checked for faded ...
Reuvan went away before Pesach and made Shimon a shliach (emissary) to be bodek (check) his home on lyl yud-daled (the night before Pesach) for chametz. (See beginning of Siman 436 and Mishneh Brurah ...
Does anyone know of any rabbinic sources (Talmudic/midrashic) that indicate that a Zav would go to a halakhic authority to be examined regarding his Zivah?