Originally Posted by Lead Cenobite
As far as I know, originally Superman finds out in the first kryptonite story in 1948, when he follows the path of the kryptonite meteor back to its source, traveling through time and arriving on Krypton as a phantom. Then in the silver age it was retconned that due to Superman's flawless memory, he could remember his short life on Krypton in perfect detail, which is probably a lot of the reason he identified first as a Kryptonian and second as Clark Kent.
Yeah, I remember that being a big part of Kurosawa's argument regarding why Superman should be the real person and Clark just a disguise. It seems that Byrne's ideas (in regards to Clark only finding out his origin after becoming Superman) weren't as radical as Kurosawa tried to lead us to believe. It seems some Byrne reboot ideas have a good bit of basis in the Golden Age version of the character.