Look at this picture carefully and try to determine where the septic field begins and ends. Look carefully... and you might just see it.
You are looking down a hill, and the slope is about 15 degrees. It is a perfect place for a septic drain field, but this one does not seem to be percolating or draining properly. Notice how dry the material around the field is, but not the field itself.
The spines of this system are running from where the photo is shot, and you can seel puffy "lines" (8 or 10) where the drain tile is located. Something is happening that causes the effluent to move upward and feed the surface plants. The bottom of the hill shows the most abundant vegetation. I would call that lush!
A septic field should not look like a "green" roof!
My recommendation: When you and your clients are looking at a house with a septic field, take the time to walk over the field. If you see defined lines, notice any odors, walk on smooshy ground or see unusually lush vegetation, like above, there is a problem that needs further investigation by a qualified septic specialist.
You know the old saying: "Effluent flows down!" Or something like that...