For security reasons, airlines are not supposed to tell you the actual loads on the planes. Sometimes you can call reservations, and they might say "it looks full", but it is THEIR interpretation.
As an airline employee, I can assure you that my version of FULL is different than reservations! If the airline cannot sell any more seats on that flight, it is obviously a full flight. However, just because you don't get your requested seat assignment, that doesn't necessarily mean the flight is full.
Loads on airplanes vary hourly. If one flight cancels- that means your "wide open" flight is now full!