Originally Posted by LonelyNinja
Still seems like a bit of a jump for Naruto to realize that Sage attacks work, since he's pretty retarded.
You've got to be an idiot.
For all of Naruto's goofiness and Armadillo dick examining, his fight IQ has always been rather impressive in serious moments. Trying to show off against Rock Lee in Part I? Yeah, he'll do something stupid, but the Shadow Shuriken against Zabuza? Kishi's ALWAYS given him moments like that, even in Part II like when he gets around his Toad Sage and Kurama incapability by using the scrolls or when he used the Transformation Technique to turn himself into a decoy Rasenshuriken.
Kishi foreshadowed this when he had Kurama inform Naruto that the Juubi was essentially composed of nature chakra. So, when ninjutsu can't work against Obito, he'd think to use Sage Mode.