Once you have your AP there is an AP section that you will be able to log into on the Disney website. Also mousesavers.com always lists the current promotions that Disney is offering.
To check availability without an AP in hand, you will need to contact a TA or WDW itself. Working with a TA or calling WDW CRO will not require an AP in hand to check prices or book the room, you will just need the AP at check in.