In this world, roughly 1% of people have actual supernatural ability (a combination of magical, deific, witchcraft, whatever). Their abilities range from the spectacular (pyromancy, flying, indestructibility) down to the quiet utility (bird poop repulsion) and all the way down to the underwhelming (a sixth sense that warns you of approaching cockroaches).
But fully 5% of people believe that they have magical abilities. They talk to plants, come back the next week, and they believe that the ones they spoke to are healthier than the others, ergo they MUST have a magical green thumb. Etcetera.
With that background, here's the scenario. Fred and Joe are alone together, no witnesses. They argue, and Fred pokes Joe in the chest and utters a curse. Joe immediately collapses. By the time medical help arrives, Joe is dead. An autopsy declares that Joe died of a perfectly natural heart attack. Should Fred be guilty of anything? Is there any way to ascertain whether Fred's curse was meaningful, short of having him attempt to curse someone else and seeing if they die?
Does it make a difference if Joe was known to have believed that Fred's curses have real meaning?