As far as i can remember back to the days where i was flying an calibration aircraft in the uk, a check was carried out three times a year.
If not calibrated, it should not be on, unless for test purposes and it should ident TST , which incidently is the same when the ILS is calibrated .
whilst calibrated it is not available for use by other than the calibration aircraft.
Monitors measure the signal strength and accuracy of the LLZ which in case of drop would then automatically change to the other transmitter, there is always two as far as i know.
Once the calibration is completed a report will be sent to the airport operator and the CAA and it will state that the ILS was within limits at the time of calibration, or not of course.
Glideslope angel could change slightly and is not nescessary 3 deg , as watertable can change and with that the glideslope angel . could be out of limits until the next calibration, without anybody knowing.
Hope that helps. this could have changed slightly as it is 8 years ago since i last flew those aircraft.
rgds