The almost mythical TV detector vans did once exist (not that many people ever saw one). They could detect the interference sent out by the electromagnets on a CRT but, contrary to the propaganda, they could never tell what channel you were watching. If you lived in a block of flats they were all but useless at working out who did and did not have a TV.

When home computers became commonplace, each with a big CRT monitor, the TV detectors were scuppered as they couldn't tell the difference between a TV and a PC.

Now that we all have LED or LCD screens for our TVs and computers the concept of a reliable detector device, able to distinguish between the two is outdated. The UK TV licensing authorities rely on cross referencing addresses with license registrations to detect possible miscreants.