If you suspect allergies are at the root of your child’s health problems, talk to your pediatrician. You may be referred to an allergist who can conduct skin or blood tests to determine what your child is allergic to. 

If you think your child has a food allergy, your doctor may also ask you to keep a food diary for a few weeks. In it, you’ll record everything your child eats and any symptoms that emerge. You may also be advised to eliminate a suspected food for two weeks or so, then re-introduce it while at the doctor’s office, in case your child experiences a serious reaction.    
 
Is it a food allergy or food intolerance?  
A food allergy can sometimes be confused with food intolerance. The difference: A true allergy involves the immune response. Having an intolerance for a food means the body lacks enzymes needed to help process certain compounds—such as the lactose in milk. When a child eats such foods, she experiences gas, bloating, stomachache, loose stools and, in some cases, vomiting. While symptoms associated with food allergies occur immediately after contact with the foods—and even trace amounts can cause problems—people with a food intolerance may not see symptoms until several hours after eating the foods and small amounts don’t necessarily create problems.