You can get the injector balance rates checked to see how efficiently they are performing. If the balance rates don't come back within spec you'll see if any of the injectors are on their way out the door.
The '03's were good, but it's been my experience up north that cold weather and sh-- ULSD fuel with modern high pressure injection rates mean injectors won't last as long as old mechanical ones will regardless of what year we're talking.
Aside from the injectors, just all the other usual stuff. I'm not sure how to check proper fuel pump operation...