Greg Cox wrote:
Spock was surprised as anybody else when T'Pring actually chose the challenge option.
While there was no reason for Spock to suspect at the time he beamed down, when the bridal party entered he could have begun to realize.
First official clue would have been the presence of Stonn. I think that it was fairly obvious that the original plan was that he was to be the champion. Everyone else in the bridal party had a role or duty. Why was Stonn walking in?
Second clue might have been to entrance of the weapons carriers, while it might be tradition to alway have weapon present, I do wonder if they are only brought if there is a anticipation of combat.
It might have not been the biggest of secrets (except to Spock) that T'Pring was going to contest the marriage.