The parts should be washed in the same way we humans wash them in normal course of events. This means that we are ought not to verify that every inch of the body has been washed, as long as we have washed them properly (the way washing is done). In case of repeated tasks (like Wuzu for example), which we are attuned to in our daily life, we ought to do them properly even if our attention is diverted somewhere else. So unless one have a certain reason to believe that they skipped some part, they shouldn’t bother about attention deficit in such tasks. Such matters when left to human nature are conducted seamlessly most of the times without any major difficulty on their part like a part of their normal course of events.
Further details can be found in the following discussions:
Discussion 62352
Discussion 59302