I don't know how things are these days, but I booked a guarantee cabin with Carnival back in 2003 and my agent told me it was pretty much impossible to get the cabin number before the cruise started. I changed the booking to a specific cabin and really won't try a guarantee again since I prefer to pick a cabin based on it's location.
Anyway, there will be someone on the pier with a manifest who will tell you your cabin number. You then get to complete your baggage tags while standing there.
There was a way to find out - known as the "Kelley Method" - where you went on the Carnival website and tried to order a gift for the booking by repeatedly entering cabin numbers until it "took" one. However, Carnival got wise to it and it won't work that way anymore. Again, this was a few years ago and I don't know if they have loosened up on divulging cabin numbers or not.