You usually need to take the cell out, unless you have one with a clear case. Typically there are unions that can be opened to remove the cell assembly. Look inside from one end and you should see a stack of metal plates. If you have calcium scaling some or all of the plates will be white. If there is not scaling then all of the plates will look metallic.
The plates can be cleaned by soaking in a muriatic acid solution. The recommended dilution varies a bit from manufacturer to manufacturer but four parts of water and one part of acid is a good place to start. The plates should bubble/foam when put in the acid and when the bubbling/foaming stops take the cell out and rinse it in water. If there is still scale on the plates this process may need to be repeated but that is rare.
Thanks for the rundown on the decalcifying the plates. I just converted to salt water and was wondering how to tell when they are clean. My manufacturer (Compupoo) recommends 5 parts water to 1 part acid for the cleaning.