I've felt weird things in my heart all my life but doesn't everyone ... or do they?
The simple answer is No. Heart disease is one of the few things that I don't have to worry about. It isn't an issue in my family, which is why I got upset when my GP insisted that I see a cardiologist due to my very low resting heart rate. I told him it was simply Athlete's heart, but he insisted. Fortunately, I was right and he was wrong.
As for how can they find out, there are wearable heart monitors these days. A friend had to wear one as he went about life for a couple of days. His doctor was also concerned about an excessively low rate, but his wasn't due to being very fit. In his case they didn't find anything, but could have.