A lot of people I know have fronted (perhaps including myself... perhaps not...), and I don't know anybody who has ever been caught out by it.
As you say, it's a terribly difficult thing to prove without them going to the lengths of knocking on neighbours doors and asking them who drives it most often. That seems unlikely to me, not least because most people wouldn't give the time of day to some unsolicited moron knocking the door to ask them such questions.
Even if a neighbour did stick you in, it'd just be their word, and for all the insurance company knows, they've just not been looking when you've been driving it - unless the neighbours are constantly peering out into the street, then their opinion of who drives it most isn't going to adequately reflect real life.
Thinking about it rationally, I very much doubt that insurers have the facilities to prove that you've been fronting, and their best tactic to reduce incidences of fronting is to put out a scare campaign to convince people that they're likely to be caught (precisely the same way that the TV Licence people operate...).