At the Polynesian, we just told the folks at the valet/bell stand. They checked and our room wasn't ready. We then asked them to alert the concierge staff that we were on our way so that they would open the door when we got there.
What resort will you be at? I think it is handled a little differently at each one since at some you can't just walk to the building as we did. Some require a key for the elevator to access the correct floor.
But regardless of where you are staying, you could always ask at the bell stand.