>The book, I mentioned above, had a very good example. You are standing some 20 or more feet away from a couple or some people who speak among themselves. You cannot hear a singe word distinctly but only the sound of the conversation. 99% of the time you can tell that people speak Polish, English, Spanish, German, Chinese, Russian etc. simple by the intonation of their language. Even though you may or may not speak or understand that language. I have experienced this situation many times.

And in some cases you don't really have to hear much - just seeing their gesticulation gives you a hint of what language they may speak :).