How does a cable company know whether or not you are receiving their signal?

Inspired by an answer to a previous question.

If a prior tenant had cable service and moved on, supposedly cancelling their subscription, yet it is still available in the apartment would the cable company know this and if so, how?
They used to use of a specialized box which used to need a card to activate and use but most televisions these days do not need the box except for the more expensive services offered.
Do they send out signals that somehow respond to specialized equipment at the home office?
Do they have trackers that go around the area and if so, how can they differentiate who has/hasn’t got service in a multifamily unit?
Do they just rely on people to be honest and report it?