How to calculate receiving shadow?

I want to find shadows that objects fall onto each other. With Cesium, I can create shadows of objects, but I have no idea how to get the shadows that fall on objects. How can I calculate the amount of shadows or amount of light falling on objects? Should I use shadow maps? If yes how can I find the shadow amount on an entity? Thanks.

As of now the shadowing system is designed for rendering and not analytical purposes. So it is not really possible to find which objects cast onto what objects or find the amount of light/shadow an entity receives, unless you modify the shadowing system extensively.