That's indeed how I try to derive an estimate from the GDS too...

If I see only full fare economy booking classes being offered still, I can pretty much guess the airline is selling its last economy seats (or even overbooking the flight slightly!), but if I see 9 (or 7 for some airlines, especially closer to the departure date) extending way down the cheaper and more restricted economy booking classes also, I know the flight is wide open and the airline is most likely not overselling their flight: if they'd do, they'd do it on full fares only, not with heavily discounted tickets and certainly not to/from Europe, as bumping a pax is extremely expensive overhere thanks to some EU regulations.