Speaking as a physician, but not an AME, lawyer, or FAA person:
If I were in your shoes, ie had been told "a doctor once said you might have asthma when you were three years old" or something, but then you spent your entire teenage / adult life never using an albuterol inhaler, never waking up in the night coughing, not wheezing after exercise, not getting bad seasonal allergies, not having secondhand smoke or other irritants give you breathing problems, not getting short of breath with common colds, and otherwise feeling like my lungs were just fine, I would think "well, that family member must have heard the doc thinking out loud, not diagnosing me."
If on the other hand I did have one or more of those issues above, or just felt like my lungs weren't quite as good as most other people I know, I'd go to my PCP, tell them the story and my current issues, and get a pulmonary function test. Mostly because I'd rather be on appropriate treatments so as not to deteriorate over time.
For what it's worth, someone with asthma so mild that they don't even know if they have it shouldn't have trouble getting a medical issued on the spot by a decent AME anyway. The worksheet for CACI (conditions AMEs can issue) for asthma is here:
https://www.faa.gov/about/office_or.../offices/aam/ame/guide/media/C-CACIAsthma.pdf
If it's worse than that, it takes a back-and-forth with FAA.