Fruits and vegetables are about 95% water, so you could do a quick measurement by displacement. To do this, you fill a jar with water to the top, put the apple in and catch all the water that spills over the top--the water that is displaced by the apple. The volume of water that spills over should be close to the volume of water in the apple. Perhaps the apple has maybe 5% less, so you could just subtract a little bit to allow for the skin and stem and stuff.

A more accurate method would be to weigh the item, then leave it in a warm oven until it's crisp and dry, then weigh what's left. The difference between the original weight, and the weight of the dry bits would be the weight of the water. It might help to slice it up for faster drying. Don't forget to weigh the item, and write down its weight, before you put it in the oven.