The basic answer is they assume that every residential address has a TV and send threatening letters to any address that doesn’t have a licence. If you ignore the threatening letters, they will send people round to knock on your door and try to see if you do in fact have a TV in the house. There were claims in the past of “detector vans”, and in the era of analogue CRT TVs there are certain things they could hypothetically detect, but they’ve never produced evidence in court from that, and it is likely the whole thing was just a made up ruse to make people think they’ll get caught, so encourage them to pay up.
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