its a test called a pft . You go to a hospital to do it. The technican askes you to blow into this tube thing connected to a computer. they plug off your nose so you can't cheat and breath through your nose. they have you blow out as hard as you can , and then you quickly inhale the biggest breath you can. the computer takes the readings and shows if you have any shortcomings in either area which may represent small airway disease (which is asthma)