Those who can't tell whether an egg is hard boiled or raw can tell the difference by placing the egg on a flat surface and spinning it by hand; a cooked egg will spin smoothly, while a raw egg will wobble. The difference in spin quality between a raw and hard-boiled egg is due to the yolk. In a cooked egg, the yolk will be solidly encased within the cooked white and will not be able to move when spun, leading to a smooth spin, while a raw egg's yolk will be able to move freely, which will cause the egg's center of gravity to shift as the yolk moves within the egg.
When the raw egg's yolk moves inside the egg and throws off the center of gravity, it will lead to a wobblier spin. This same principle can be observed when the person testing whether the egg is cooked reaches out and gently stops the egg from spinning by lightly touching it. A cooked egg will stop smoothly and, if it isn't knocked off kilter by the hand, will not wobble much when stopped. By contrast, the raw egg will wobble quite a bit as the loose yolk continues to move inside.