If you decide that you saw the dunk
(along with the other coach), just make a mental note to yourself of the player(s) number(s). Then let your partner know that you have a T to handle. Walk to the head coach of the team that dunked and calmly let him know that his player dunked the ball and that the game will start with a T. (You may want to let him know that he is now seatbelted for the remainder of the game).
When the table crew returns, let them know what you have called.
There is no reason to make a big display about the situation. You do not need to blow your whistle and give the "T" sign when this situation occurs. Just report factually and calmly to the coach and ignore the language that he uses to explain to the player how much trouble he will be in at the next practice.