Remember, "retired" is a specific term referring to product availability from LEGO itself. Once they officially retire a set, none will be sold online or made and shipped to retailers. You can get the retirement date from a site like brickset.com. However, any out in the wilds will still be there, and as HK mentioned, poor sellers can languish long after official "retirement". A good example of this is Lone Ranger Silver Mine Shootout, which has an official retirement date of Dec 2013, but is still available at many retailers, live and online - nearly a year later. Another "popular" example is Battle at Weathertop, which was retired from LEGO but was still found in stores for months after.
I don't believe LEGO officially retires a product LINE in swoop, sometimes it takes forever for them to officially retire a set that's long gone. It just kind of happens when all the sets in a line eventually sell out/are stopped being produced. Monster Fighters trickled, Lone Ranger trickled, with a few sets still being produced after a few others retiring, and Galaxy Squad is working the same way.
For example, Star Slicer (70703) is officially retired, but most of the Galaxy Squad line is not. Galaxy Squad seems to be following the pattern of Monster Fighters: They become TRU exclusives after other retailers stop selling them. And we "unofficially" declare a line retired when it becomes obvious they aren't creating more sets for it, which seems to be the case with GS.
For example, if you go to Haunted House on SHOPLEGO, and click on the "Monster Fighters" tag, it will bring you to the main page, telling you Monster Fighters as a theme is no more. Same with Lone Ranger. However, doing the same with a Galaxy Squad set will bring you to 8 remaining sets, 2 of which are call for availability. So with a couple sets already retired, no new ones being released, it's safe to assume the line is being retired. If you look up Lone Ranger Train Chase, it still says "Retiring Soon" even though this set hasn't been avail for sale since December 2013. We all know it is "retired".
Using Monster Fighters as an example, I suspect TRU will try to blow out the remaining Galaxy Squad stuff during this holiday season. they had a lot of sales on MF things during last years season, and very few sets made it through. I also have a sneaking suspicion that the Castle theme will replace Galaxy Squad as the TRU "exclusive" line going into next year.