I've had a lot of luck with this:

me: Guess what someone just asked me ten minutes ago.
her: what? me: Do you speak english? her: really? me: Yes, can you see anything about me that would make you think that I don't speak English? I guess some people think I look northern european. (I am tall and blond w. blue eyes) Her: (she gives her opinion) me: After I replied that I do indeed speak english she asked me which way Broadway was. I told them; they thanked me and I answered with "Bitte schön!"