Power Inspector has the ability to display the state of any EditorWindows that are currently open in the Editor.
Open the extended view menu by holding down the control key while clicking the view menu button. You will find a list of all EditorWindows that are currently open in the editor under the menu category EditorWindow Debugger. Click any of the items listed to inspect the state of the corresponding window.
EditorWindows and Debug Mode+
You can do so using the Debug Mode+ button found on the header toolbar.
Inspecting Static Members of EditorWindows
It is also possible to view the static members of an inspected EditorWindow class.
To do so, open the extended context menu for the drawer by right-clicking the drawer header while holding down the control key, and select the menu item Inspector [ClassName] Static Members.