During an autopsy how can the doctor know that the cause of death was from caridac arrythmia?

History. There are no direct pathological findings in an autopsy of cardiac arrhythmia. We use the ECG monitor feed or automated defibrillator information from ems if patient is attached to one before death. However, if that objective data is not available, I use the diagnosis "no anatomical cause of death, presumed arrhythmia" and then try to explain what history or findings point to arrhythmia.
It's tricky. Death usually results in the end from a rhythm problem, but one due to a genetic mutation or medication may be subtle. Coronary artery disease, emphysema, and a variety of heart structure problems are common and commonly kill suddenly by rhythm problems, and finding one of these is sufficient for me if there's no suspicious circumstances though nowadays i check most folks for drugs of abuse.