How?does a medical examiner determine that the cause of death was a cardiac arrhythmia?

Exclusion: All deaths are ultimately from the electrical system of the heart stopping, but if someone not on drugs simply falls over dead and the autopsy & labs show no clear fatal lesion, i look for entities that cause rhythm problems such as coronary atherosclerosis, emphysema, or structural defects. When not present, look for a channelopathy (old ekg's, gene studies). That's a start -- wishing you comfort.

Great question: He or she may be unable to, unless there is recorded ECG data before death!