How did my doctor know i have asthma? is there an obvious sign?

History & exam: The diagnosis would be based on your history and the doctor's findings from examining you. He will also obtain a lung function test on it to see if there is any airway obstruction and may repeat the test after you have been given a bronchodilator such as albuterol. For questionable cases, a methacholine inhalation challenge may also be considerded.

See below: Often history gives the diagnosis away. For asthma, unless you are having an exacerbation, there are no physical signs typically. A pulmonary function test would be the first test to diagnose asthma. It would likely be normal if you are not having an exacerbation. Then a methacholine challenge test would be ordered to confirm.