I believe that as long as you have a credit card attached to your room, you'll have express check out added to your room. You can ask the CM checking you in just to be sure. The way it works is that morning that you're set to check out, a bill of your room charges will be delivered to your door. If there isn't a problem with the charges, you can just leave and the front desk will bill the charges to your card.
I usually go to the front desk the night before and ask for a copy of the bill ... sometimes I find there isn't enough time in the morning to look over the bill and even when I have the time, there can be a long line at the front desk.