The ads said they could tell if you were watching telly. I suspect they detected aerials, but that was in the days of Cathode Ray Tubes, and maybe you can detect them being on, I don’t know.

You can absolutely detect a CRT from outside. A CRT is basically a small particle accelerator with a magnetic deflection system, inside an unshielded plastic (or wooden, if you go back far enough) box. Of course, it will need to be turned on, just the presence of a TV doesn’t show up. It’s probably a fair bit harder to actually detect which house the signal comes from, but you can solve that with a big enough directional antenna. With analog TV, you might even be able to detect which channel they’re watching, based on the exact frequency, which makes it easy to tell a TV from a computer monitor too.

Spotting a flatscreen TV is probably a LOT harder, and distinguishing between a TV recieving a signal and computer monitor seems (to my lay skills) pretty much impossible.