Finding leaking and overflowing gutters when it is raining is a lot easier than determining if they will leak or will overflow if it is not raining. One thing that inspectors look for when it is not raining is impact marks on the ground under areas of overflowing and leaking. Often times these will show up as hollowed out areas or as washed gravely areas. In the first picture note the "moss" ring around the washed area indicating that this area stays wetter than the surrounding ground.
Sometimes it is decay or rot where a deck surface doesn't have a chance to dry out due to continual dripping of the gutters. Like in the picture below, where the leaking "sectional" type gutter continually leaks water due to improper sloping of the gutter (gutter has standing water).
These sectional type gutters are much more prone to leaking than ones with no seams. Leaking at the joints can cause hidden decay/rot to the fascia where they leak as well as damage surfaces below. When leaks occur over walkways, icing and moss growth can create slippery conditions leading to falls. In the next picture we can see the obvious potential for issues from the leaking gutter. Some leaks like this might not show up at the time of inspection. In this case I was "lucky" enough for the stairs to still be wet.
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WA State, Home Inspector Advisory Licensing Board