If the front end of the car has the 2009.5 styling, it's either a 2009.5 or a 2010. If it has the older front facia... it's mislabeled and is 2009 or earlier.
Easiest way to know for sure if it has StabiliTrak is to have an independent dealer do a build order/sheet on the car. Give another dealer the VIN and have them check for FX3, that is the code for StabiliTrak.
ANY G6 with FX3 has StabiliTrak, and ANY G6 that doesn't have FX3 lacks StabiliTrak. No exceptions.
The rest of this was to try and figure out an easier way than having a build order made up for each car you're interested in, as many dealers often leave off StabiliTrak from their feature list (especially online). Some also accidentally list it on cars that lack it.
Of course, the real culprit is GM here... all cars should have StabiliTrak. Unfortunately, they only got around to realizing this after Pontiac's fate was basically sealed (for now anyways).