Question: When we are playing for money or points, who is responsible for tallying a player’s score? How do I know what I have to pay the winner?

Answer:  The winning player (winner) is responsible for announcing the value of their hand.  In the situation where the winning player forgets to include the value for a jokerless hand and another player points it out, the winning player is paid what they initially declared.   

Source:  Mah Jongg Made Easy (2020 page 30 para. 11)