Unless you have bad eyes, what better way is there to determine if a picture is sharp?
Seriously though, you can see the difference. Put on a 50mm lens and take a photo with a shutter speed of 1/500s and another with 1/20s. That should give you a sense of sharp vs slight blur.
The single most important element to getting sharp images with the D800 is shutter speed. Play by the 'rules' of shutter speed and focal length (or be conservative and shoot faster than 1/focal length) and you'll do well in most situations with the D800. Frankly, I haven't found it that much more difficult to get good results as many reviews suggest. Maybe people have been lazy with their technique.