I don't think the platform triggers the delete event. Technically the object isn't deleted, it is no longer kept in memory. They have the same end result, but as far as I know we don't have an event that runs on cleanup in memory.
The most reliable way is to look at the cleanup debug messages from the platfrom.
You can enable TRACE logging for the LogNode: Core. The Core log node prints how many object it garbage collects, if objects are being garbage collected that lognode will print how many objects of each type are being removed.
Keep in mind though that objects are only removed from memory if the are no longer accessible through other objects in memory. During garbage collection the platform looks in memory. If the object is still associated to an object in use it will not be garbage collected. For example if your objects are associated to the currentUser, which is always kept in memory since he is logged in and the user info is stored in the session. If you would associate your non-persistent objects with the current user, they will never be garbage collected (until the session is destroyed).
But this same applies to something as simple as when you still have a dataview open in the browser, all associated non-persistent entities will stay in memory as long as that dataview object is used in the browser. When the client releases the dataview object, that object, and all associated objects will be garbage collected.