Hey Jessi,
Tests to diagnose asthma include:
Spirometry Test: This test measures the narrowing of your bronchial tubes by checking how much air you can exhale after a deep breath, and how fast you can breathe out.
Peak flow: A peak flow meter is a simple device that can be used at home to help detect subtle changes before you notice symptoms. If the readings are lower than usual, it's a sign your asthma may be about to flare up. Your doctor will give you instructions on how to track and deal with low readings.
Methacholine bronchial challenge: If you have asthma, inhaling a known asthma trigger called methacholine will cause mild constriction of your airways. A positive methacholine test supports a diagnosis of asthma. This test may be used if your initial lung function test is normal.
Recently I have come across a test called exhaled nitric oxide(eNO) test which is used to diagnose and monitor asthma. It measures the amount of gas called nitric oxide you have in your breath. If your airways are inflamed â a sign of asthma â you may have higher than normal levels of nitric oxide
Hope you find it helpful.