The rule is if you watch something that is on TV in the UK at the same time then you need a licence. So you are OK watching recorded stuff on youtube. You could watch cbc.ca live as they do not broadcast in the UK. You could not watch CNN.com live as they do broadcast in the UK.How do they know what kind of TV people have - do the TV police conduct raids? What if you don't own a TV but watch everything online? Do you also need a computer license and a phone license?
Oh, I forgot other licenses we have here - hunting and fishing. Fishing licenses can actually be used as valid ID for voting.
As far as them knowing, they have had this story about detector vans driving around and working out who is watching TV from the emissions from the antenna. We do not really know if that is true. What we do know if that they get lists of people who do not have a TV and pay people on commision to hassass them, including going round to their houses and pretending they have the right to come in and look around your house (which, in the absence of a warrant and the police, they do not).