Whether your buying a new home or you have questions about your existing roof there are some visual issues you can look for. 

The shingles, for starters, are the most visual part of your roof. When the shingles are old or go bad a few things can happen. They can become loose or cracked. Missing shingles are a huge factor in the condition of your roof. If you have missing shingles the underlayment or wood can be exposed, letting moisture into your attic. Wavy or curled shingles are also an issue, this could mean that there isn't proper ventilation through your attic, causing your shingles to age prematurely. Also if you look into your gutters and see shingle granules, this means the shingles are old loosing their protective layer and could also lead to leaks inside your home. Other visual issues would be any cracked caulking, worn vent pipes or moss growing on the roof.  Moss growing on the roof is a sign that the wood underneath the roof may be decaying.

Visual roofing issues that would be seen on the inside of the house are dark ceiling spots, which indicate a leak into the home through the roof. Check for any dampness or wet spots around your fireplace or any pealing paint on any corners. If you have skylights, the most common visual issue would be pealing or bubbling paint or water marks around the boarder of the inside, If you see any of these warning signs, it is recommended to hire a roofing professional before the issue gets any worse. Unrepaired roof leaks will only further the issue and potentially lead to mold growing.