How do you check for soft or muddy ground on a backcountry landing?
Usually, it's a combination of making a couple of low altitude passes over the landing site and visually observing the conditions of the terrain in the area and personal experience selecting a site based upon the knowledge of the local terrain and soil, recent weather conditions, local flooding, ground vegetation, etc.  It's a bush pilot skill based partly on intuition as well as instruction from senior airmen with experience flying in the area.