They don't know. The people loading your luggage onto the ship don't bother to check there handy "list of the worlds fine wines" before putting your luggage onto the ship.
They are probably more concerned with how much you bring. A couple of bottles is no problem. A case may be pushing it. It depends on their mood. If you try to bring the wine to the dinning room, they will charge you a "corkage fee" of $15 to consume it in the dinning room. UNLESS you bring bottles of stuff they sell on the ship and then they can't be sure if you got it in a gift basket or not... So if you bring wine, try to bring one from the wine list.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007