It's all about eye contact, real Londoners will go to incredible lengths to avoid any sort of interaction with strangers. When forced to, it'll be overly polite and with a certain mutual derision that's kind of hard to explain. As for correctly identifying someone who doesn't exhibit that behaviour -- all the scottish ones tend to reek of booze, the french people who are around all wield baguettes, the welsh are making weird sounds instead of talking and when you can understand what they're saying it's usually uncomfortably correct things about the other uk rugby teams... The few americans can be deduced by mass... At least, that's how I do it :P