Yes, they can tell if you have had an irregular heart rhythm by doing an EKG, or Halter Monitor (some 24-48 hours). Perhaps doctors have rolled their eyes as you are young. Most dont expect younger people to have heart conditions...but, it happens. It did to me.
Do you have a family history of heart disease? This will increase your odds of having a heart condition. Drinking, smoking, diet, any number of things could be the cause of what is going on. So basically you will not know until you just get a full work up done.