kakashi said that in the chunin exam and not sure if he knew and dont think he would have showed any facial expression if it was the real naruto
just some information
The Byakugan is only used in the noble Hyuga Clan. It is said that the Sharingan is descended directly from the Byakugan. The Byakugan allows the user to see the critical areas and where the chakra is stored in his/her opponent(s). Very rarely can a Byakugan user see the Tenketsu (Chakra holes) and neutralize them, literally stopping their chakra flow and eventually killing the opponent. Neji Hyuga is one of the few who can do that. One direct hit with the Jyuuken style will completely stop chakra flow into the targeted hole. The Byakugan also allows the user to see almost 360 degrees or panoramic view seeing things from all direction. Around the 3rd vertebrate is the only area where there is a blind spot. There are four main users of this very dangerous bloodline and they are: Neji, Hinata, Hinabi, and Hiashi. There is much debate on which is stronger: the Byakugan or the Sharingan.