We all know that CDs don't live forever. Often loss of data is predictable when the disc is starts to take longer to read, but these changes in speed are often enough too small to be noticed or something else (like the overall system load) is taken responsible for it.
Therefor it is good to have a little tool that analyzes your discs properly so that you can make a backup in time. Such a tool is cdck. It's just a small command-line program, but as many Unix-tools it is a specialized tool which does it's job right.
It reads the disc block by block and measures the time it takes to read each block. Based on these statistics you get a summary about the current quality of your CD or DVD. If you like you can even output a graph which can then be displayed in GNUPlot.