When I purchased a Canon 7D I decided the first thing to do was check the focus accuracy. I found an excellent article by Keith Cooper of Northlight Images which set out very clearly the technique to use which requires a suitable pattern be displayed on a computer screen.
The camera then has to be set up on a tripod so that it is centred on the pattern and exactly square to it. Manually focus each lens on the pattern and then gently press the shutter button half way to engage the autofocus; if the lens refocuses adjustment is required as set out in the camera’s instruction book. Happily both my lenses were spot on.
The image below clearly shows the additional rings which are the moiré patterns generated through interference between the pixels of the computer screen and those of the camera.