Just to clarify, you want to know what SmartObjects are being used throughout a K2 Workflow? Unfortunately, workflows do not generate a list of SmartObjects that they use whenever they are being designed. In order to figure out which SmartObjects are being used, you will likely have to go through the workflow manually to find out.
Another suggestion, although not tested, is to perhaps use the Package and Deployment tool to determine the dependencies.
Make sure that the 'Automatically include dependent items' is checked; and see which Smartobject is returned when all of the dependencies are loaded.