AnandTech - The iPad 2,4 Review: 32nm Brings Better Battery Life
"There's no known way to tell whether you're getting an iPad 2,4 vs. the older iPad 2,1 without opening the box. The 2,4 unit I ended up with was made in China, ruling out manufacturing region as a way of telling. The external box looks identical, as does the device itself."
Going to become a guessing game.