Just wondering. would a doctor know by listening to my lungs with a stethoscope if there is asthma.

Not specifically: Asthma is a pattern disease with many features that can overlap with other conditions. A person with true asthma may also be between events, with no lung sounds different from normal. The wheezing often cited as a typical symptom can occur in other conditions. Without a review of history, exam & possibly provocative testing I would not label someone as asthmatic.