You can tell if someone was alive or not when they went into the water according to something to do with their lungs and the water in them, or not in them. I think there are other signs, too. Like when people drown, something in their throat flaps shut. If they drowned, and there are no injuries, someone would have to have been in the water holding them down.
But as to how they know if someone slipped or did it on purpose, I'm not sure.