A few months ago I was looking into making a user interface with the draw3d mission event handler. I was looking into using keybinds to interact with the menu and found that they were, more often than not, not so user friendly. Instead I opted for a way to interact with the menu by looking at the item you want to select. This is a gif of the final product.
The code I used to detect the angle you're looking at can be used for a lot of other things such as bullet prediction and a more accurate object placement system.
Here is the code.
Here is a video I have made on the subject.
https://www.youtube.com/watch?v=Ffz7odUWKGI