You can check availability on http://www.seatcounter.com .

Airlines that pre-assign seats do make some seats not pre-assignable ("under airport control"), so that they are only assignable at check-in. Presumably, this is so that they can arrange for late booking groups like families with small children, persons with disabilities and attendants, etc. to be able to sit together without having to move too many people out of pre-assigned seats. It also hides overbooking situations from customers. However, those without pre-assigned seats are at higher risk of being IDB than those without pre-assigned seats, if all other factors (e.g. status, booking class, check-in time, etc.) are equal.

Try checking in on-line as soon as the on-line check-in window opens.