Try to look at the seat map it will tell more at least in Business and First that most have preassign seats, but even that some time not correct.
I had flight show F2 A2, the seat map and online check-in was with 3 seats from the 16 occupied the flight left with 3 on F
but in other hand I add more than once that the seat map was with many free seats and at the end the cabin was full so not sure there is a real way to see how Full flight is...