Even if you don't have US cell service, you can download the Delta app and use it for free with the GoGo Wi-Fi on ORD-ATL. I recommend checking your itinerary on the app en route to find your arrival and departure gates. You can then check out the Sky magazine at your seat to see how far you'll have to go in each terminal. I do recommend quickly checking again on the screen that will be above your head just as you deplane to ensure a gate change hasn't occurred.