TV detector vans certainly do work
They contain sophisticated receiving equipment that use adaptively filtered EM receivers as well as other signal receptors to detect the leakage of EM and other energy that all video displays radiate, and are capable of comparing the distinctive spectrum of that energy to that of a live TV transmission.
The equipment is extremely unlikely to give a false positive, and will work just as well and in the same manner whether the screen is CRT, LCD or LED, and whether it is being fed from an analogue, digital, terrestrial, satellite or a live TV Internet feed. Large screens are however a lot easier to detect than small screens, and handheld devices are usually not detectable at all unless conditions are exceptionally favourable. It has a fairly big detection range and can be operated from outside the premises.
The equipment in question is part of the van's operator, and is also known as "eyes" and "ears". Look at the windows or any residence at night and the chances are that in a large percentage you will see a characteristic brightening and dimming of light around the edges of the curtains - coming from the screen of a TV set in the room. Look at a block of flats and you will be able to identify all the flats in which the same TV channels are showing as they brighten and dim in perfect synch. If compared with what is currently being broadcast on the most popular TV channels, it will be pretty easy to see if the pattern of bright and dim coincides with what's being broadcast on one of those channels. If so, you could double-check at the appropriate moment by having a listen at the letterbox to see whether the familiar sound of, say, the Eastenders theme tune can be heard, but that is hardly necessary.