What is a dead giveaway on SW that someone is English despite them trying to hide it ?

Politics. They have very similar and condescending opinions about Americans. You know its a Brit when they start talking about how Conservatism must be defeated in America for it to move forward.

Also not an exclusive trait to the British, but a sure sign is a complete lack of understanding regarding guns in America and complete exasperation with the fact that guns haven't been made illegal in America like where they live.