Puzzle from Lecture 1

In lecture 1 I gave the following puzzle:

Suppose a group of friends are playing a game of Where's Waldo? where the first person to spot Waldo gets $10. However, the remaining players want to continue the game after the first player spots Waldo (with the second player finding Waldo maybe getting $5, and so on), so the winner shouldn't actually reveal where Waldo is!

How is this possible? That is, how can the winner convince the others that it actually knows where Waldo is (and thus should be paid the $10), but without spoiling the game for the rest?