I had a 96 chevy 6.5 diesel at one time (I know that's mine bad luck),but I use to check the lift pump pressure by connecting a pressure gauge to the fuel filter discharge hose(water drain) and opening the valve while the engine was running and check the lift pump pressure.I know the discharge hose is bigger and would need some kind of adapter to the pressure gauge.
Can this be done on the cummins 04 ?

Just a thought