In a forest, you usually can find your way by looking at the side of trees since the most moss accumulates on the north sides of trees, especially in temperate regions of the Northern Hemisphere.
Moss prefers dark, damp areas and thrives when those conditions are met.
Because the sun shines from the south in the Northern Hemisphere, the north side of the tree will tend to be the shadiest and also the dampest. This encourages more moss to grow on the north side.
In a particularly dense forest where everything is shaded, moss will grow equally everywhere since no side of the trees gets more sunlight than others. In addition, this logic would be reversed in the Southern Hemisphere.