Go on ExpertFlyer and set up a Seat Alert (you can do this for one flight at a time with a free membership). In the process of setting up the alert it will show you a seat map for your flight and indicate which seats are taken vs. which are merely blocked. That way, you can both tell if they are already taken or not, and if they are merely blocked, you can set up an alert (alerts for a specific seat will cost $1 I think) for when they free up. Or, you can just set an alarm to try to grab them at T-24, which is presumably the most likely time they would come free if they are blocked.