I want to know if there is a GUI.Box under the mousepointer. How to do that?
I think I need input.mouseposition but don’t know how to find the GUI.Box.
You could either put some information in the tooltip for each Box, and read GUI.tooltip if it is set (see the link for tutorial on how), or you could keep a collection of rects for your boxes and check if any of them contain your cursor.
Btw: Use Event.current.mousePosition for GUI space coordinates.