Because he and the Kyuubi are the only living beings in the world who have seen the Shinigami's face without getting their entire souls taken by him.
Orochimaru survived Shikki Fuujin and only lost his arms. But he saw what the Shinigami looked like.
That is how he knew which mask it was. It's impossible for anyone else to tell.
Orochimaru survived Shikki Fuujin and only lost his arms. But he saw what the Shinigami looked like.
That is how he knew which mask it was. It's impossible for anyone else to tell.