First of all anytime you plan to get off the ground be prepared and don’t work alone if possible. On a low slope roof I generally climb on the roof if possible. Getting a top view of the shingles and roof penetrations is best but if you are afraid of heights you should stay on the ground.  Here is how I would do it.  Start on the ridge and work toward the eaves looking for missing shingles. I also look for open or improper flashings which are likely to leak. Cupped or curled shingles are a sign of problems as well. There may be loose rocks on the roof near the eaves as a result of ice damming which  makes the roof very slippery so be careful. If the eaves troughs contain a lot of rock from the shingles you know that the shingles are failing. Overall the entire roof should look the same. For the inexperienced I suggest looking for areas that are different. Usually a roof will fail in certain areas such as the south side first. If something doesn’t