Personally. I either
1. Manually focus using live view and 10x zoom, including DoF preview. I use this a lot on the tilt/shift lenses, which are my main landscape lens. The focus confirmation helps here also.
2. Focus off something in the scence at appropriate distance (Either autofocus, or manual using focus confirmation), then check the scale on lens just to make sure of DoF
Regarding Diffraction, it is a strange beast. Been doing some research on this latley.
On my 1dsmkIII, anything beyond f10, will suffer from diffraction limiting the effective resolution. This is a law of the physics of light pretty much.
In my research on the net and some testing, I concluded that when the size of the airy disk gets bigger than twice the size of a pixel, (twice is an accepted value, on a bayer array), then detail will be lost due to diffraction.
This has nothing to do with print sizes, circles of confusion etc, I'm just refering to CAPTURE resolution and amount of detail captured in a shot.
When using the scale on the lens to set the DoF (effectivley, focusing a the Hyperfocal distance), it is a good idea to use an appeture a stop lower than what you are shooting, as these scales are overly optimistic in what is 'acceptably sharp'.
For the mathamatically inclined, A good site to play with the number is Cambridge Color. http://www.cambridgeincolour.com/tutorials...-calculator.htm
Try changing the setting from 'manufacturers standard' to 20/20 Vision. The distances change considerably !.