Personally, I assume I do not have a code if I haven't gotten any notification. Others have posted that they call and ask. I would say if you are calling in to make reservations, it wouldn't hurt to ask if you have one.
I did at one point email WDW just to ask about codes. Their response was that it is done geographically. I've only gotten a postcard once, and truly didn't know what it was! (pre-DIS days!) It didn't fit into our schedule, so I didn't use it.