The BBC claim they can detect if you're watching, but this is of course, bullshit. The TV receives broadcast media - it doesn't send anything out. The only way they can catch you is in the act, if you leave your blinds or curtains open and they can see the TV playing live.
The main tactic they use to make people pay the license is constant intimidation and threats. There is a form you can fill out to clearly state that you do not use a TV for any of the purposes which require a license fee, but this has little use as the bullies will still come knocking even if you fill it out.
The main thing people need is awareness that they do not need to pay the license fee to merely own a TV for watching Netflix or anything not covered by the license. If one of their cronies comes knocking, you tell them to get lost. If they come again, threaten to report them for harassment. If they take you to court, you explain that you don't use the TV for any of the covered purposes and the BBC pays your court costs and leaves you alone after that.
Old school CRT TVs and monitors most definitely rebroadcast what they are displaying [1]. Modern TVs still do, but not at the same power levels. I would guess that a tempest attack on a modern TV is very difficult. I don't know if the BBC has ever used any of these techniques. But it's not impossible.
This has never been the case.