What principle is used to determine if a game is a game of chance?
The Dominant Factor Test is the standard used in U.S. courts to determine if a particular game qualifies as a game of chance. Most games feature elements of both chance and skill, but the dominant factor in determining the game's result must be chance in order for it to be considered gambling. So if an amateur boxer challenges Mike Tyson to eighteen rounds, the dominant factor determining the outcome would not be chance but skill, and it would not be gambling to wager on it.
SOURCE: Dominant Factor Test | Wikipedia.org