If you tell someone in a conversation that the Union Flag is only called the "Union Jack" when it's flown from a ship and that you can recognize when it's flown upside down (thus signalling distress) you automatically become 62% British.
I knew that about the Union Flag/Union Jack, but I'm afraid I'm not quite British enough to notice if it's upside-down or not. I did go to preschool in London and lived there for one-and-a-half years, if that counts for anything... Never did pick up an accent or any particularly British idioms or spellings or other "clues" of that sort, though.
Just curious...