The answer comes down to this: Whatever is safe. The best way to inspect anything is to get right up to it. But not every roof should be walked on. Some are in bad condition, overly steep and would need special equipment to climb, and sometimes the weather (such as when it is raining) all become safety factors. When a roof is not safe to walk on, how is it inspected?
Extra thorough is the best practice if a roof can't be walked on. Spending extra time in the attic, at the eaves of the roof (if not raining, etc), checking every nook and cranny possible.
The point is to give the best inspection possible. So if it is raining, the roof is damaged, or some other condition prevents walking on the roof, don't worry. We spend extra time thoroughly inspecting every aspect of your roof possible.