There are gray patch cards to measure such things. The go from black to white in steps. You set your black to 0 on your scope and white to 100, or to 110, 120 to check the cameras white compression. Then measure your steps.
Perhaps I clarify. DR by itself (say 5 f/stops) does not tell you much. Because you must also state what settings are used on HPX250 - for example KNEE = AUTO, LOW, MEDIUM (for that particular DR).
Finding DR reminds me of the Zacuto shootout with film and DSLRs... Grey scale testing is around 24 min. I'd like to see how the 250 stacks up. http://philipbloom.net/2010/03/24/shootout/