A detailed history and physical examination (including accurate height and weight) are usually sufficient to establish a cause. Over 80% of cases are due to insufficient caloric intake from exogenous causes.
History should focus on breastfeeding technique, frequency, and duration, or amount of formula and method of preparation.
Observed feeds can often be useful to identify problems.
If an endogenous cause is suspected, specific lab tests or imaging studies may be helpful to evaluate for metabolic abnormalities, the presence of infections, malignancy, or anatomic malformations. Additional testing should be ordered on a case by case basis and is only recommended if a specific etiology is suspected.
History should focus on breastfeeding technique, frequency, and duration, or amount of formula and method of preparation.
Observed feeds can often be useful to identify problems.
Failure to thrive 
GERD
Stridor
IBS
Stridor 
Food allergy
Hiccups
Malabsorption