If you cannot get it ahead of time, IAD
has loads of EasyCheckIn machines at the main terminal. Unless the flight is full, you can chose to change your seat, and they will provide a seatmap on the screen, and you just click the open seat you want.
Or, after it looks like most people have boarded, and they are going to close the door soon, walk upstairs and check for any open seats/rows. I've found that most UA
flight attendents don't care if you swap seats, as long as they're unoccupied and in the same class.