According to InterNACHI Standards of Practice, we are not required to walk on the roof, but we normally conduct our typical roof inspection by walking on the roof's surface (if deemed safe by the inspector) in what we call the "random walk" methodology. This method of inspection is not intended to cover every square inch of the roof's surface, nor will it. Further we could not recreate the route of a random walk even if we tried to. We do arrive at an overall impression of the roof's condition developed during this random walk inspection and extrapolate it to the entire roof's surface.
If any discrepancies are in fact identified, it is recommended that to accurately determined the scope of the actual discrepancies, as well as any cost of correction, you consult with a licensed roofing contractor. Not all roof will be walked, dimension, slope, weather, etc may allow the roof to be accessed.