Starting at the top, with the roof.
Take a walk around your property to inspect the roof. Take a camera or binoculars and stand as far back as possible to see more of roof surface, now you can zoom in to see the ridge caps, roof valleys and other areas.
If there is moss starting to root along shingles, or debris covering roof it is time for a cleaning.
If moss is growing on the roof, it will start to damage the shingles, the older the moss the more damage it will cause. Roof treatments and cleaning should be considered.
If possible, have a look into the gutters from a window or ladder, if safe. If the roof is covered in debris, then there is a good chance your gutters are also. Clean gutters are the best way to keep water away from unwanted areas. A good rule is to clean gutters every 2 years.
When walking and inspecting the roof and gutters, look at all wood trim, fascia and gable ends.
Notice the paint condition, look for wood rot or excessive staining. If the wood is rotting repairs will be needed, if the paint is flaking, peeling or faded with exposed wood it is time for paint.
Protected surfaces will last much longer and eliminate costly repairs.
Now look at all walkways, driveways, foundation and landscaping.
If the areas are green, mossy, stained or slimy, it is time to clean. Sealing these areas will help to keep then clean and safe.