I commute on Airtran a lot. I have not checked the seat maps on the reseration web site lately, as they used to be widely inaccurate. The best way to know is to see how cheaply they are selling tickets. They have a multi-teir deal where the more full the flight the more it costs. If you cant see a flight it is sold out. Even the internal company site changes drastically. Case in point I listed myself standby the night prior and it went from showing 10 empty seat and 3 standby--- When I showed it was oversold with 37 stanbys.

Might point is have a back up