The best way to determine if an athlete is consuming adequate kilocalories is to

The answer to this question would be: track changes in weight over time.

When a person consumes an adequate amount of calorie, the weight wouldn't be changed. If the calorie is not enough, the body will burn the fat stored to fulfill the energy required, result in reduced body weight. If the calorie is too much, the body will store the excess calorie as fat, results in increased body weight.