I thought you were American. Surely you are American? Aren't you American? You must surely be an American.

One might say, "I take it that you are American". "You look like an American". "You must be American". "No doubt you are American".

-"Are you American?" -"Are you an American?" -"You look like you're American, are you?" -"I saw you over there and thought you could be American, are you?" (The last two sentences aren't direct translations, but they get to the point).