Quote Originally Posted by The Heretic Azazel
I'm pretty sure Sasuke said that in the filler Tea Country Race arc, when everyone was on the ship.
It was at the beginning of the battle in The Valley of End. Anyway it really doesn't matter where it was, I know it isn't canon.
Quote Originally Posted by KrayZ33
I think the right question would be "why should it be possible for him to tell the difference?"

from what I know, the clones are physically alike, chakra wise too and even able to think on their own
Because its the sharingan.