Most Germans don't believe that I am an American because when I speak German, I don't have the American accent. And moreover, because I speak German... they find that to be quite odd. Only with one of my German friends do I converse in English. And that probably has more to do with his having to speak mostly in English at his job. So he's used to it. However, if a German sees or "hears" that you are not a native speaker, he/she will probably ask where you are from. And if you say the U.S., they'll probably just switch over to English, thinking that it will be easier. I recommend that if both you and the German want to practice your respective language skills, ask the the German to speak in English while you respond in German. You'll each get a better idea if you are saying the right words or not.