Peer review allows players to gain an understanding of another player’s potential ability in order to form a reasonable basis for supporting or disputing scores that have been posted.
It is the process by which either a player’s score or Handicap Index® can be confirmed or challenged. While the Handicap Index calculation has built in safeguards to protect against handicap manipulation, the human element is equally important to ensure fair play.
Peer review is normally conducted by someone playing in the same group during a round, or by someone who has knowledge of a player’s demonstrated ability and can reasonably verify or challenge the player’s Handicap Index.
To facilitate the process of peer review, scores must be posted as soon as possible after completion of the round. Challenges and disputes should be raised with the player and/or reported to the Handicap Committee of the golf club where the player is a member for consideration. (Rule 4.4, Rules of Handicapping)
Source: Rules of Handicapping FAQs