The trouble is that sometimes they look wrong because they ARE wrong, and they ALSO look like plausible dialect. An example I recently ran across, in one of my mother's books: "Are you through your breakfast already, dear?" Yes, it's possible that's an idiom I'm not familiar with (or haven't ever noticed in speech). It's also possible the author (or the typesetter) simply left out a "with" after "through." (In this case the author's dead, so I can't ask her.)

I did an editing certificate program some years back, and we spent a lot of time on when to query and how to phrase queries appropriately. It's not easy, even for nonfiction.