The easiest way is the engine data plate... 325HP is the 04. 5, 305HP is the 04.
Exhausts can be changed, cats tend to run away, etc. Build dates are tricky, when the "conversion" year is being done, things can get mixed up for the first few months. Probably the absolute best way would be to have the dealer run the VIN... that will tell you what engine (I believe they term it "600TQ Cummins" in their build sheet).