How can I tell if someone died from a heart attack?

I'm writing a short story about unexplained series of deaths. Later it's revealed that these deaths were caused by heart attacks due to… being scared to death or dying of great fear. Is it possible to tell if someone died of a heart attack by looking at them and examining their body or would an autopsy have to be performed. In an autopsy, how could they tell that someone died of a heart attack? Any help would be much appreciated!

Thanks!