I suppose if I was trying to check a simulation's results I would first look at the pressure/temperature profile in the pipeline. Break the pipeline up into small enough lengths to assume the pressure is constant over each segment (or the average of the inlet and outlet pressure is representative). Use these pressures, temperatures to calculate the density in each segment and with the line lengths, the pipeline inventory.
Repeat for the final conditions. The difference between the two is your line pack.
As Morten says though, how much difference are you seeing?