The other way to tell is if you see people using the near side lane on the motorway. That’s illegal in the sarf I think.
I’m a born and bred southerner who regards my time living in Loughborough as my ‘Northern Chapter’ and that is sadly true.
The reason for the squiggle is that it’s easier to be northern (as denoted by a funny accent) if you live west, so Birmingham is more northern than Peterborough, for example. For the record, the correct dividing line is from the Wash to just above Gloucester. Anything above the Manchester-Leeds motorway zone is ‘Proper North’ and at some point between there and the borders you are into ‘Here be Dragons’ territory.
Norfolk, East Anglia etc are largely their own little bit, like Wales.