There is always a fullname but if a file is new and has never been saved then there won't be a path.
Code:
Sub SavedNotSaved()
If ActiveDocument.Path = "" Then 'file is new and not saved with filename
Debug.Print "New unsaved doc"
Else
Debug.Print "It's been saved"
End If
End Sub
In an unsaved document, fullname returns the name in the banner eg Document1