Referees’ calls in sports are determined by a combination of rules, training, and situational context. They must be well-versed in the specific regulations governing a sport, assessing each situation with fairness and impartiality. Their decisions can be influenced by factors such as player behavior, game dynamics, and available technology, like VAR in soccer or instant replay in American football. Ultimately, a referee's judgment plays a critical role in maintaining the integrity and flow of the game.