http://www.nhlbi.nih.gov/health/dci/Diseases/Asthma/Asthma_Diagnosis.html

Medical history:  Doctor will ask about a family history of asthma or allergies, as well as one’s own personal asthma symptoms (how often do they occur?  Specific time of the day or night? What triggers the symptoms?)

Physical examination: doctor will listen to one’s breathing as well as look for other physical symptoms of asthma and/or allergies, such as wheezing, runny nose or inflammation of the nasal passage, or eczema.

Tests: spirometry – this tests lung function; how much air one can breathe in and our of their lungs, as well as how fast one can breathe out.  Additional testing may include: allergy test, airway sensitivity test, chest X-ray, and determining if it’s actually a disease with the same symptoms of asthma.