Hello,
the method of Jim Jones is the best. I take a cover glas of a 6x6 slide cut to size. It has to be placed exactly on the film guiding rails. Onto the front side toward the lens I make some marks or dots with a black Edding. The shutter is opened at B with a fixable cable release, aperture fully open. I take an infinity target (antenna or pylon) at least 500 mtr away. The focus is checked with a very strong loupe (10 - 20 x or a reversed strong wide angle lens). The image of the black marks has to be sharp together with the infinity target. Of course the camera has to be fixed on a table or tripod.