How can the doctor tell if your child has asthma?
  Your child’s doctor will ask a detailed description of your child’s symptoms, history of asthma and other allergic conditions in the family. If your child is 6 years old and older, the doctor may consider a special test (spirometry) to measure how much air your child’s lungs can hold.

A skin test for allergy may also be done. If your child is below 6 years of age, the doctor will have to rely on detailed information from you about the type of symptom(s), its frequency and may require a period of observation before a diagnosis of asthma can be made.