If it was built after 1/1/04, then it's an '04. 5.
The easiest way to tell is to read the label on the driver's side of the plastic valve cover. If it says "305" hp or less, it's an '04. The '04. 5 had 325 hp.
Also, look at the radiator fan shroud. The '04. 5 had it attached to the engine.
Bob