Colour is a good one… Canadians say go to hospital instead of go to the hospital…so their semi easy to spot…
then there are the usernames: if the username doesn’t sound American, then it’s most likely not an American:
American-sounding usernames: bob, jake, bill, kate, jessica, sirBoring, america
non-American-sounding usernames: not bob, not jake, not bill, not kate, not jessica, not sirBoring