If you had said at the start what Charles has guessed you are trying to do (in Post #59) it would saved over 50 posts. If the receiver is a superheterodyne you MAY be able to detect the local oscillator frequency. This will be the received frequency (315 Mhz) + or - the IF frequency. As the customers may be using different models of receiver you will have to find the local oscillator frequencies of all the models of receiver being used. You will then need a sensitive receiver capable of being tuned to each of those frequencies or a spectrum analyser that covers the frequency range. I did not post on this thread before as like others I had no idea what you were trying to do.

Les.

 

How close can you get to the power boxes?
A loop antenna tuned to around 315Mhz and a SDR in a laptop may allow you to see something by looking at the waterfall display, but if the receivers are just a TRF design, I think you will not get anywhere. Try this in the workshop.
I would think if the power reads drop well below normal, an official visit to check the meter "to see if it is faulty" (not officially looking for a defeat device) is in order.
Then, if a defeat device is found, very heavy fines or power disconnection should follow.