In the English dub of the episode The Risks of the Reanimation Jutsu, I noticed Sasuke refers to Tobi as Tobi. But Sasuke shouldn't know at this point that he isn't Madara, as far as I know, so it seems odd. And it occurred to me that I can't exactly remember whether Tobi introduced himself as Madara to Sasuke (I think he did, but I'm not sure). Additionally, Sasuke doesn't seem to react much when Kabuto mentions the real Madara. So what does Sasuke know at this point? Who does he think Tobi is?
During the Shippuden Itachi vs Sasuke fight, Sasuke figured out that during the Uchiha Annihilation, even Itachi couldn't defeat the entire LPF himself. He knows someone helped him, and Itachi revealed it was Madara, the fake one atleast. Sasuke also knows Obito/Tobi has a Sharingan, but has no way of conforming that Tobi was the one who helped Itachi, due to his mask. And when Itachi mentioned Madara, Sasuke wanted to kill him, as if he didn't know he died ages ago. I'm pretty sure after Itachi used genjutsu to show Madara, Sasuke knew he was dead. So Sasuke could've speculated that Tobi was some random dude who had Sharingan,or a random guy who happens to go by Madara, an Uchiha that Itachi couldn't kill, or, using his keen intellect, he could assume that Tobi is the one who helped Itachi with the Uchiha Annihilation. Tobi also tells people "Call me Tobi or Madara, whatever you like." So maybe they had Sasuke choose a random one of the two names.