What you heard would be こうなるって知ってた(ん)でしょ or something along the line, wouldn't it? It's basically for confirmation, as in the first examle in tokkyo-san's explanation, and when it's said with the falling intonation, it can have a (soft) accusing tone, something like "I'm sure you must know ~. Don't you?". This expression is not for politeness or guess, so it's not interchangeable with そうだ, らしい or ~と思う. (I suppose the speaker is female, since males usually use だろ instead.)