I will try to save you some time because this can be a time consuming exercise. Over at the Leica User Forum do some searches on focus calibration, focus testing etc. A recent thread had a leica focus chart as an attachment. An older focus chart was in come usage when the M8 was released. Finally there is a new product "lens align" that offers a decent alternative. Google and you should find a link to the mfg who has a nice short video.
While it is possible to adjust an M8 s rangefinder by yourself .....its more complicated than it looks ..again discussed in detail on the LUF.
Keep in mind that three things can be off....(1)camera rangefinder (2)lens itself (3)photographer error. First make sure you are using a diopter to correct your vision and a magnifier 1.25x or 1.4x for lenses over 50mm. This was the best advice I received and I only need mild (150) lenses for reading and use a 1.0 diopter .
You can do a simple test by laying a ruler out and focusing on a specific number. Try it 3-4 times and you should get a sense of the focus point. A good tip is to look at the numbers in front and in back of the focus point...think 40% in front and 60% in back as in focus depth of field .
There is a lot more to this but its enough to answer the main question..is the camera or the photographer .