My testing was very similar to the method Linwood described.
First - you need to understand that the camera has two different AF systems - phase detect through the viewfinder and contrast detect in live view and mirrorless cameras. Phase detect is faster, while contrast detect is more accurate. The problem is with phase detect AF while contrast detect with Live View is accurate. Adjustments such as fine tuning auto focus or errors live the D800 AF problem only involve phase detect AF.
Second - there are valid reasons why you still get out of focus images toward the edge of the frame. The outer sensors are not cross sensors, and the center sensor is the best of all the cross sensors which makes the center more accurate. AF is not a perfect process - you will have AF misses some percentage of the time. Light levels affect AF performance. And it is possible to have soft iamges due to lens issues such as softer outer edges of the lens which may be normal - or not - and decentered lens elements.
If you are going to test, you need to be perfectly perpendicular to a good AF target. I use the LensAlign target. You should also take a number of frames with each sensor you are testing - at least 5 frames defocusing between frames to make sure you are getting consistent errors.
I used shutter delay set for 3 seconds and a tripod for all test images. I used both a 50mm f/1.4 lens and a 24-70 f/2.8 lens set wide open.
My test was a little simpler. I used the center of the target only. I took 5 images using the center sensor. Then I took 3 frames using the center sensor and Live View. Then I took 5 frames of the center of the target with the far right sensor, and then 5 frames with the far right sensor with a Live View series for each. The Live View images confirm your camera and lens are capable of capturing sharp images.
My center sensor was excellent for all 5 frames. The far right sensor was slightly soft but still acceptable. The far left sensor was significantly out of focus on all five test images and there was no variation. Live View images were sharp in all cases.
The test confirmed I had a problem, but I rarely use the extreme sensors. So I waited nearly 9 months until my warranty was near expiration before sending my camera for service. I wanted to avoid any risk of losing my camera or doing anything to decrease AF accuracy because the center sensor was so good and so sharp.
If the outer AF sensor is soft, you must have the Live View images sharp to confirm a problem. If the Live View images are also soft, your problem is likely with the lens and not the camera. If you are getting variation - some sharp and some not - repeat the test. The problem is not an intermittent problem and intermittent misfocus reflects a testing error.