The post is chock-full of clever advice and comparison pictures - for example, when you connect the real white iPhone 4 to your computer, its icon is that of a white iPhone, whereas the imitation's icons are of the black one, since that's what's inside. If you don't have a computer handy, you can just look inside the headphone or data ports - if they are black, then the thing has been converted, but if white, the chances that you scored the real deal increase exponentially.
If the audio jack and the data port's interior is black (left), then it's an aftermarket conversion kit; if white - it might be the real deal
What we found most interesting in the whole post, though, was the comparison of the light leak problem in the white iPhone 4 with the black version. While this issue still exist, it seems to have been brought down to a minimum, and has finally complied with Apple's rigid quality standards, which has obviously led to Cupertino greenlighting the mass production of the white iPhone 4.
Light leakage from the white iPhone 4 compared to the black version still exists, but is brought down significantly from the initial batches last year
source: IT168 (translated)