come on, didnt you watch "the blind bandit" episode??? They answer that question in that episode and if you havn't you definitely shold cuz it's one of my all time favorite episodes. Basically, the person that aang saw in the swamp that looked like toph really was toph. And remeber the flying boar that was also what aang saw? that's the symbol of the bayfung family (toph's family) and it's that vision that aang had that helps him find toph.