A TV license is required to watch broadcast television, so the number of channels they could be watching is finite. Even a muffled signal detected from a vibrating window (via laser microphone) could be compared to what is currently being broadcast by each of the channels (give or take a short processing delay) and any match found.

Nowadays it's mostly database enforcement. You obviously don't have to care about addresses that do have TV licenses so you just go looking for people with various TV subscriptions (Sky, Virgin Media) that don't have a TV license. It used to be done by the retailers submitting addresses of people who bought/rented TV equipment (TV, VCR, etc), any that didn't get (or already have) a TV license would get a visit shortly afterwards. Also people who watch programmes 'live' on-line (IP -> address conversion thanks to the DSL providers).