the flyback frequency was quite distinctive
They used the IF or intermediate frequency generated by the TVs tuner. This was 39MHz above the signal from the channel being received.
For instance, the bottom channel #21 was at 470MHz. So they would use a receiver tuned to 509MHz and a directional hand held aerial to pinpoint a set to gain their evidence.
These days just say nothing (they are recording every word) and shut the door.