Go to the airline's website. There is always a link for seat assignments or seating charts close to the flight you'd consider. I think Expedia's, etc., shows the links too but don't forget you normally have to pay additional fees for booking through those sites.
Our Dec flight is virtually empty, it was switched to Song. But unlike other stories, ours went smoothly. No changes in the schedule were made and we're on row 2!
But being it a fact that the flight is in Dec, close to the holidays and school vacation closings, I do expect it to fill up within months. I suppose people are waiting to see how the economy shapes up but it always worked for us to book several hundred days ahead of time. Each year I did, the price never went down below what it was 300 days ahead of arrival or departure!
And there are yet more advantages to planning early: You can motivate your children to save money for the trip and pay off as much as you can, to the point that when you get there you virtually have the expenses of the stay and do not return to see the bills after the trip!