You don't. It's a supposition that should be based upon either previous knowledge of an arrhythmia in that patient, or anatomic/histological, or previous electrocardiographic criteria that predispose to arrhythmias (as you point out).
If she had, on autopsy, a cardiomyopathy that predisposed her to arrhythmia (e.g. hypertrophic cardiomyopathy, arrhythmogenic right ventricular dysplasia, a dilated cardiomyopathy, etc.) then this would be a reasonable assumption. Coronary disease would be very unusual and rare in a healthy young woman of her age, unless she had some type of congenital coronary anomaly. In any case, these would be found during autopsy.
There are conditions predisposing to arrhythmia, such as long QT syndrome, Brugada syndrome, active myocarditis, etc., but it's a reach to make these diagnoses without evidence (e.g. previous ECGs, autopsy histological data showing myocarditis, etc.).
The bottom line is that if there was adequate reason to make the diagnosis of 'arrhythmia' as the cause of death, then they should release that data and put this all to rest - for the families sake mainly.
Absolutely true— excellent review of the reality of the time when this occurred. Verification of a myocardial infarction- as to the post MI enzymes, well.. can’t recall if these were done and how long she had been deceased. The official COD was head trauma injury, her head hitting a desk, as there was trauma identified. As for the “falling”...? That would be difficult ex post facto without prior medical records/EKG.. etc.