What to actually know
Most rural mountain homes run fine on a well and a septic system for decades. The trouble isn't that wells and septic are bad — it's that buyers assume they work like city water and sewer, and find the gaps too late in the timeline to do anything about them. Colorado makes this its own subject: water here is allocated by prior appropriation ("first in time, first in right"), so water rights attach to a priority date and a beneficial use, not to whoever owns the ground. A well permit from the Colorado Division of Water Resources authorizes a well to use a limited amount of water under stated conditions. That is a different legal thing from an adjudicated water right, and a rural buyer can encounter one, both, or neither.
So the job isn't to scare you off a cabin on five acres. It's to verify four things in time: what the well permit actually allows, what the well actually produces and whether the water is clean, whether any shared well has a recorded agreement, and what condition the septic is in. The genuinely common, avoidable problems are a permit-versus-use mismatch, an unverified yield, a missing shared-well agreement, and a septic surprise. I flag each one, put it in your due-diligence window, and bring in the driller, the septic inspector, the county, and — where water rights are in play — a Colorado water attorney.