As a parent, how does one know when they've been successful raising their brood? When a child emerges from adolescence without a police record? When the children graduate college? When they land their first job? When the first grandchild is born?
Those are the rewards of successful parenting. They highlight milestones of years of great influence that led to those significant achievements. The parent that has a great relationship with a responsible, mature, adult child knows success. This was accomplished by day after day of responsible parenting, leading by example, and imparting influence that made an impression.
While the job really never ends, success leaves clues!