Sound could be an option.<p>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.<p>Nowadays it&#x27;s mostly database enforcement. You obviously don&#x27;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&#x27;t have a TV license. It used to be done by the retailers submitting addresses of people who bought&#x2F;rented TV equipment (TV, VCR, etc), any that didn&#x27;t get (or already have) a TV license would get a visit shortly afterwards. Also people who watch programmes &#x27;live&#x27; on-line (IP -&gt; address conversion thanks to the DSL providers).