I think he started to make connections from before Euphie's death. I can't pin point a certain point in the series when he might have begun to think it, but probably during that fight when Lelouch gave Suzaku that power supply for the Lancelot, he probably started to see the similarities, if he hadn't before that. But even so, he didn't want to believe that his theories were true, because him and Lelouch were best friends. So he made himself try to ignore it all.
I don't think he KNEW though. When Lelouch's face was first relieved, you could see the surprise in Suzaku's face. Then everything he had begun to put together that made him think Lelouch = Zero in the first place was confirmed to be true. I don't think anyone told him before that, Cornelia couldn't have because she said "she couldn't remember" and was too out of it because of the Geass and her wounds. And because of the flashback, we can see it wasn't V.V.
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I don't think he KNEW though. When Lelouch's face was first relieved, you could see the surprise in Suzaku's face. Then everything he had begun to put together that made him think Lelouch = Zero in the first place was confirmed to be true. I don't think anyone told him before that, Cornelia couldn't have because she said "she couldn't remember" and was too out of it because of the Geass and her wounds. And because of the flashback, we can see it wasn't V.V.