If you want to know for sure, for the unique engine in your vehicle, the best way is an oil analysis. You take a sample and send it to a lab that runs tests, then returns all the info about the oil to you. There are a lot of different things you can see in a report, from water/fuel/insolubles in the oil, to flash point and additive content.
I send a sample each oil change to Blackstone Labs, and they have recommended a 10,000 mile oil change interval on my 5.3. I was changing my oil every 5,000 miles before, so even with the cost of the analysis I come out ahead since I end up doing half the maintenance of before, and I have the added benefit of keeping an eye on wear metals in my oil.