Two indicators come to mind. First, Korea used to be known as the Hermit Kingdom. Today, that title accurately describes North Korea, a country with limited communications links which suggests that they would need a lot of "outside help" to pull off this stunt.
Second, the depth and breadth of the attack appears to be so massive that it almost looks like everything on their servers was copied and carted out. If they actually did this from outside, the Russian hackers must be green with envy.
An additional thought: If you have this kind of capability, why blow it on a small target? For comparison, look at the Allies' preparations for D-Day in 1944 and notice how we cloaked our capabilities and methods. As I'm writing this, Leo Laporte, the Computer Guy, came on the air making the same points. Way to go, Leo.