The EKG shows the electrical conduction through your heart. Sometimes small changes in the EKG can be a sign that a heart is getting sicker. Once in a while, your doctor will ask you to wear a Holter monitor, which is like a portable EKG that can monitor for arrhythmias happening throughout the day.
An exercise test may also be used to look for arrhythmias during exercise. It can also be helpful to check if your heart is strong enough to keep you active.