Not sure about decapitations, but ash piles are activators linked to the corpse. The corpse is hiden by the disintegration effect. I suppose you could determine if a body is disintegrated by it's alpha value. Anyway, searching for the activators (ash piles (two types), ghostly remains (three types) and ice piles), that are linked to the corpse should be easier.
EDIT: Now that I think about it, Decapitated heads are head parts. Surelly they will have some kind of link to the body and surelly also they will be few of them, to search for them by base ID.