Mandarin-Cantonese is very interesting and a unique (to my knowledge) example where the same written language can be completely different to two different people.

I don't buy the argument of just asking the speakers. There are cultural, political, etc. reasons people may think things which don't conform with reality. Many Hindi-Urdu speakers get insulted by the reality that the languages are pretty much the same because they don't want to identify with people from another country their country is constantly at war with.