You turn the engine off and let it settle back down before checking it. The easiest way to refill it is to fill the filter before installing. Then add oil for a total of 14 - 14.5 quarts (this includes what you've already added to the filter). Start it up and let it run for a minute and shut it down. Let the oil settle back down and check it. It's a 15 quart system so you'll be close. Top it off and your done.
Joe