Hi Luigi Gaeta,
The server-side Delete event fires when an Item is deleted as well as when it is Transferred (because it is first deleted in the source ListBox and then transferred to the destination one), so there are no built-in means of distinguishing between the both Delete operations.
Fortunately the client-side Delete event fires only when an Item is deleted via the Delete button. So you could use this event to raise a flag indicating the meaning of the Delete operation.
So you could add a HiddenField on your page with a default value of "true":
and handle the client-side Deleting event:
Finally in the server-side Deleting event handler you will know whether the Item is being Deleted or Transferred:
sender, RadListBoxDeletingEventArgs e)
// Item is being deleted.
Please note that you need to re-set the value of the HiddenField so that the approach works for all subsequent operations.
All the best,
the Telerik team