Go to the Carnival website and log onto your account. Then click on Modify Booking. If you scroll down, your stateroom number is listed there. We booked a guarantee room and found out 2 weeks ago what our stateroom number is. Good luck to you!
If you booked with someone else, you won't see modify booking. It only works if you booked with Carnival. The only trick I've heard of is people ordering a gift online and then plug every room number there is into the room # box and keep trying until it finally accepts the number. But that could take hrs!