If you are speaking to the focus problem in the early release D800/E bodies ….its in the far left most focus point . You can put a bottle on a table …move the focus point to the left middle sensor and establish AF . Check the result ..its obvious if its off. Nikon has a firmware fix .
If you are checking for general AF accuracy and want to “tune “ your lenses to the body . Read up on the AF adjustment in the users manual . You can buy a “alignment tool “ that can be helpful …I ve done it so many times I just use whatever is on the table . You do this with the center focus point . I adjust it and then test both near and at 2 meters .
The alignment tools can be more accurate and if you are doing a bunch of AF lenses …maybe worth it .
A quick test I use …is throw a hat onto the grass and focus on the emblem ….you can t miss the target and you have the grass to see whether the focus is front or back .