Short answer, no, there never has been a reliable method, in the early days when there was a low density of TVs in roads (only a few houses had them) it was possible to pick up the radio signal generated by the TVs flyback transformer, and triangulate the position of the TV, that was the early detector vans, by the 70s that had become unworkable due to the high number of TVs and the detector vans were just empty vans used as scare tactics.
Todays Tvs don't have high voltage circuitry like CRT TVs and are suppressed to prevent interference, so there is nothing to pick up.
It has been mooted that TV licensing are using a technology that picks up the light colour showing round the curtains and comparing that to a colour signature from broadcast TV, but with 20+ years servicing TV sets, I'd say that is yet more hogwash.
Its not just the BBC though, the licence fee covers the IBA transmitter network for terrestrial TV as well as the BBCs cut, so all TV is covered by it.
The satellite and cable providers are also covered because they by default show those terrestrial channels.