Capture the center of rotation in 3D mode

I need to capture camera information when rotating the scene with the default CTRL + MOUSE_LEFT_DOWN inputs.

Specifically, I want to capture the beginning of this scene rotation (moment when both CTRL and MOUSE_LEFT_DOWN are active) and the end of the scene rotation.

The reason I want to do this is to display some sort of graphic element at the 3D position of the center of rotation when (and only when) the scene is rotating.