These days it's just done via the database they have, you're the exception if you don't have a licence so it's easy for them to check on those people. You also have to gove a name and address when buying any TV, Freeview box or whatever over the counter and they'll get you that way too; presuming you don't just lie - Mr L.P. Blower, 29 Acacia Road etc.

They have controlers who walk from door to door and check the households with their database. If they find a household that is not listed in their databse, they will ring and try to check. They are known for being aggressive and also to play with foul tricks and wrongly quoted legal rules on occasions, just to get access to the flat and see if there is a TV or radio around somewhere.