AIUI (from distant days working at BT) the thing they were looking for was the EMR of the CRT flyback transformer. But it could only be detected if the television was towards the front of the house (not behind too many internal walls) and only on the ground floor.

That method has got no hope of detecting a modern LCD/OLED/Plasma display.

The majority of enforcement is database driven:-

Royal Mail database of addresses - addresses of current tv licenses = huge list of possible offenders

Now cross check database of possible offenders with databases of: * Sky or Virgin Media subscribers * people who have recently purchased or rented appropriate equipment (televisions, VCRs, PVRs, etc) * etc