Several cruise lines use FLL as a port. It's up to the harbor master to assign pier space, so there's no definite answer that you could take to the bank.
However, taxi drivers KNOW these things.
If pressed, just look for the ship with the big "X" on it and you will be fine. If there's more than one "X"-rated ship and you can't see the name on the hull, just ask a porter (there are always tons of porters milling about) if it's the right ship.
Don't worry. You'll be on board the right ship before you know it !!!
If you are taking a taxi over to the pier, the driver will know what/where your ship is. I was quite amazed when flying into Ft. Lauderdale, you could actually see the ships from the plane right before your landed. The port is about a 10 minute ride from the airport, very close.
Celebrity generally, not always, departs from the new Pier 18... mid-port. At every entry to the Port, there is a sign board listing every ship berthed and the piers at which they can be found. No need to worry!