Not all aircraft have them, but eye-reference guides are very useful for pilots to be able to set up their seating position correctly. Every flight deck is designed in a way that the pilot should be able to see and interact with every vital piece of information/device without having to move their head. They will obviously need to move their eyes, and swivel their head sometimes, but setting their eye position is a great way to make sure that the pilot can see everything that they need to see, including out of the window.

To use the red and white balls, the pilot simply sits so the red ball completely obscures the view of the white ball behind (or vice-versa on some aircraft, depending on colours), and their eye position should be correct.