Hi, John. I questioned an asthma diagnosis years ago (no testing done) and decided that I had really been having anxiety attacks, as it only happened when showering in my too cold bathroom. My most recent visit to a pulmonologist resulted in a new round of pulmonary function tests and a diagnosis of severe COPD and mild asthma. Asthma! So I went looking on the internet for how asthmas is diagnosed. The following is an old article, but it is so clear and so detailed in how the testing is done (consistent with my recent tests) that it answered all my questions. From this article, I would guess that asthma is caused by many different things, since it is inflammation of airways -- more a symptom than an independent disease. At least that's my nonmedical understanding of it.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2774375/