There are breathing tests that can be done to measure your lung volume. Asthma can be hard to diagnose. Not all asthma is the same. Most people wease. Some people cough. This can be much harder to diagnose and there can be many trips to the er and many doctors listening to lungs before it's diagnosed.
My first son's first attack was very bad, intensive care in a major children's hospital. My second son they had trouble diagnosing. Then I was diagnosed. Even I didn't think it was asthma but after about the 5th trip to the hospital while pregnant and not being able to breath I was diagnosed. My third son they diagnosed right away because of family history of weird coughing asthma.