There is a simple asthma test. You will blow into a machine called a spirometer, both before and after taking an inhaled medicine. This test is positive for asthma if the result after the inhaled medicine is 20% higher than it was before the inhaled medicine.
There is a complex asthma test that is the gold standard for diagnosing asthma. This is called a methacholine challenge. You will blow into a spirometer before and after each increasing dose of an inhaled medicine. This test is positive for asthma if the result after the inhaled medicine is 20% lower than it was before the inhaled medicine.
By an asthma test, what exactly did your doctor mean? One of the common tests which m-i-g-h-t have been meant is a spirometry test before and after bronchodilator (such as albuterol). If your breathing numbers improve after the medication, it is some evidence of asthma. Another test that your doctor m-a-y have meant is a methacholine challenge test, where you are tested before and after you've inhaled increasing amounts of a chemical to determine whether you are asthmatic (see thread below on methacholine challenge).
In my experience, the DOCTOR'S office generally schedules all such tests--insurers won't pay for tests scheduled by patients! At least the doctor's office makes the initial arrangements & then the patient works out the date & details with the lab. You need more information as to the exact name of the test your doctor wants you to have.
Starion