There is a lot of information everywhere about how American English vocabulary and pronunciation differs from British one, and from Australian one, etc etc etc.
OK, I understand:
He said the elevator, he must be American.
He pronounced “water” more like “wader”, he must be American.
So it is easy to tell by listening. What about reading?
Would this happen? Situation: A British person is reading an article online and thinks, “Hmm, the author must be American because he used the grammar we don’t use here in Britain”. (of course I mean when there is no obvious American vocabulary in the text)