Using the list sidebar to order objects
The list sidebar shows you every object on every layer of the current canvas, in front to back order. To open it, choose List from the View menu. If you have the outline sidebar open, you can also click the button at the top of it to switch to the list sidebar.
Layers are listed in front to back order, and inside each layer is a list of objects in front to back order. You can drag objects into different ordering or into different layers. You can also use the buttons below the list to move the selected objects forward or backward one step at a time, or all the way to the front or back of their layers.
In the General preferences, you can choose to make the list sidebar show the unique identification number for each object, which can be useful for scripting.