I imagine checking ILS accuracy by measuring ILS signals at various points along the approach path is routine maintenance, although I don't know how it is done. How often must this kind of check be done to ensure the ILS is still accurate enough?
The FAA has a Flight Inspection Manual which describes the checks that must be done and how often. Table 4.1 shows that an ILS must be flight checked every 540 days. Section 15 describes what happens during one of these checks. It's quite an extensive inspection!