With all the new pathfinder stuff being released every month...how do I know what is legal for "Pathfinder Society" play and what isn't?
Chapter 13 of the Guide to Pathfinder Organized Play contains a list of what is legal for play in Pathfinder Society Games. An updated copy of that PDF is released occasionally which will contain updates of new resources for play. So keep an eye on updates to that document and you can keep up with what new things are legal for play.
Right. Until a new release is listed in the Guide to PFS Play, nothing from that source will be legal. Also remember, just in case you or others reading this are not aware, that only material published by Paizo will be legal for PFS. So if you are hoping for some other awesome Pathfinder book put out by a different publisher to be made legal, you, and all of us, are out of luck.