THAT is untrue and total myth. IF you only watch DVD's or use a games console with your TV, then you DO NOT NEED a TV licence. You DO NOT need one for simply owing a TV!!!
IF you actually read the TV licensing website it tells you this quite clearly!
If you don't own a TV then you don't pay the TV Licence.
As to the asker: They have teams that come and check and use address databases and trackers that can tell if someone in the home or building is getting a TV signal.
However, most of the time they only go on what address isn't registered with them (for example, I have a TV licence in MY name for our address, however, when my partner bought a new TV, they sent threatening letters saying the address did not have a licence - their explanation? Because we live in a flat, and it might be my flat mate or neighbour!) Idiots LOL
They also have a problem with flats, because many people move in and out so quickly and move on!
Anyway, if you really want to know, go and Google the TV licensing and you can have a little look at the FAQ's :0)