Keep a diary 1
Because infants need almost three times the amount of energy as adults (in calories per kg), it is easy to understand how quickly infants can fall behind in growth. Keeping a diary of the type and amount of food a child eats over a three-day period is one way of quantifying caloric intake.
1. http://www.aafp.org/afp/2003/0901/p879.html. Accessed December 2014.