Since Itachi Uchiha always said complete truth, is it possible that he himself knew the truth about Tobi being Obito and not Madara?
Itachi clearly knew that Obito was the true leader of the Akatsuki, and he also knew of and feared Obito's (Sharingan) abilities, hence placing his own Mangekyo Sharingan on Sasuke and setting it so that Amaterasu activated when in the vicinity of Obito's Sharingan. Additionally, Itachi also had knowledge of his grudge against Konohagakure, which is why he confronted Obito/Tobi/Madara before the Uchiha clan massacre. However, Itachi never clearly states his knowledge of Obito's true identity, and also, Obito states during his first encounter with Sasuke that Itachi clearly knew more than he let on. It is never stated if Itachi knows that Tobi is Obito, but he clearly knew a lot about Obito.
Itachi definitely knew that Tobi was an Uchiha, but there is little evidence, if any, to say that he knew that Tobi was Obito. Why I think that Itachi would know that Tobi wasn't the real Madara was because the amount of chakra emanating from Tobi, when the real Madara was present on the battlefield everyone knew about his presence.
Itachi is the one of the most smartest people in the anime. Itachi knew it about Obito, from Kakashi, when they was in one team in ANBU - The story of the Kakashi's Sharingan. Itachi knew it about the moment when Kakashi cut it off the hand of Deidara. All of Akatsuki knew u it about this situation...and if you think more about that, from the story from Kakashi about his Sharingan, Itachi is enough smart to known about Tobi true identity. He will understand That, if Kakashi have the same time space from Mangekyo Sharingan, like Obito, they have same eyes...or just Obito is Tobi. Itachi isn't foolish. Itachi was think about that things... That 's the reason about amaterasu in Sasuke' s eye, after Itachi's dead.