There is no checking other than a database.
They stopped trying to 'detect' years ago.
Nowadays they use a big database of addresses and cross reference it with which addresses have a license.
Those that don't get letters and visits.
The only problem is their letters are very badly written and their visiting staff seem to be vetted to ensure they are twats. (Probably send them to traffic warden school)
I have a license as I think it is great value. Hell, I'd pay it for I'm Sorry I haven't a Clue alone.