There isn't any way in wotlk (at the moment) to tell which debuffs belong to you. Blizzard made it such that all debuffs now show the remaining time on it irregardless of who casted it. Currently on live realms, only your own debuffs show the remaining time on it, which is how an addon can tell which buffs belong to you.
was this a change in the last beta build? because i am pretty sure it was working before that.