First run this code into a Workspace:
Gtk default useHalos: true
This will cache all created GtkWidget objects. Now you can open a simple window with widgets inside of it and run this code:
Gtk default enterHalos
Now do Ctrl + left mouse button click and an inspector (ore more than one) should open on the clicked widget.
When you don't need halos anymore just type:
Gtk default leaveHalos
In the next version, probably, this feature can be improved. Right now it's just a funny test.
Also the inspector is not complete, i just added a few property specs. It really needs to be improved a lot and also to be more automated instead of creating a specification for each GObject.