Airlines tend to open up all inventory levels (unless the flight is way oversold) the week before a flight departs to allow people to make changes to their existing reservations. Since the advance purchase requirement can't be met, there is little risk for the airlines since only the more expensive fares can be bought and anybody wanting to make a change to their return flight is able to do so if they pony up the change fee.

The only true way to see how full a flight will be is to call up the airline and ask--even at that, the airline will only be able to tell you how many revenue seats have been sold.