> watching BBC and not something else

You're not allowed to watch "something else" either - it's a national tax on televisions, not a BBC subscription charge.

Historically the detector vans tuned in to the local oscillator frequency generated by the television's "Heterodyne" tuning circuits. In order to tune to different channels, the TV generated different frequencies and used them to shift the broadcast an intermediate frequency to where they would decode it. If you know the intermediate frequency used by the TV (I believe they all use the same one), and can measure the local oscillator frequency, you know the channel being tuned to. You may well still know the frequency being tuned to with a digital TV, but each frequency now carries a multiplex of several TV or radio channels.

https://en.wikipedia.org/wiki/Intermediate_frequency

> Unless they peep through your windows?

If you search|scroll down the TV Licence article for "In 2013 it was revealed..." these days they can literally match flickering light seen through the window bouncing off the ceiling against currently broadcasting BBC | Channel 5, ITV, etc broadcasts.

See: Optical detectors

Freeview receives data on the same 4 core broadcasts* that were used for analog, and in fact, freeview channels remain mux'ed in a way that BBC produced content is isolated to the 2 muxes that correspond to the old BBC1/BBC2 split.

So yes, it's still perfectly easy to find the Intermediate Frequency oscillator for a freeview receiver.

* During the analog/digital crossover period, the DTV signals were, obviously, at an offset from the associated analog UHF channel, since analog and DTV could not physically occupy the same frequency, but the fact remains that the DTV muxes were associated with one of the 4 existing channels (Ch 5 didn't have an associated DTV mux afaik)

edit:

here's a list of the mux allocations, you'll notice that all BBC programming is allocated to the BBCA and BBCB muxes, even though there's more than 4 overall muxes available these days.