When Deidara was in that forest and new about Sasori's death before fighting against konoha guys ( minute 01:10 [link] ) he said: "He rants about "art being beauty that lasts forever" and then he dies a second later! He had too much fight in that form, despite of how it exposed your weak spot".
How is that Deidara knew about Sasori's weak spot?. Maybe Sasori just commented it to him someday or Deidara just realized about it because he saw it with his own eyes somewhere for some unknown/random reason ( *fan-service thoughts*
), but welp, if Sasori never took out his cloak to use himself in a BATTLE since he fought against Kazekage... it's so difficult that Deidara could've managed to see it with his own eyes unless Sasori opened his cloak for some reason and showed it to him personally. I'd say Deidara only saw his face and Sasori confessed he had a weak spot (?). But welp, since manga is also full of contradictions/mistakes everywhere... everything is possible 
so we shouldn't think so much anyway.
Nobody can know when Deidara knew about Sasori's true appearance since it's not officially explained anywhere yet.
I WOULD LIKE to see that scene someday, but, as I said, I'm not going to build up my hopes because I constantly get disappointed when I do it, so I don't think we'll see it in episode #310 either (even if I would like to). At least I hope that today's episode is awesome, since they are involved
How is that Deidara knew about Sasori's weak spot?. Maybe Sasori just commented it to him someday or Deidara just realized about it because he saw it with his own eyes somewhere for some unknown/random reason ( *fan-service thoughts*
), but welp, if Sasori never took out his cloak to use himself in a BATTLE since he fought against Kazekage... it's so difficult that Deidara could've managed to see it with his own eyes unless Sasori opened his cloak for some reason and showed it to him personally. I'd say Deidara only saw his face and Sasori confessed he had a weak spot (?). But welp, since manga is also full of contradictions/mistakes everywhere... everything is possible so we shouldn't think so much anyway.Nobody can know when Deidara knew about Sasori's true appearance since it's not officially explained anywhere yet.
I WOULD LIKE to see that scene someday, but, as I said, I'm not going to build up my hopes because I constantly get disappointed when I do it, so I don't think we'll see it in episode #310 either (even if I would like to). At least I hope that today's episode is awesome, since they are involved
Who knows, perhaps beyond all that bickering about art, they actually might have been so close that Sasori eventually shared his secrets to Deidara 
But afterall they have been partners for years so I guess it's somewhat expected that they know a lot about each other.
But afterall they have been partners for years so I guess it's somewhat expected that they know a lot about each other.