To get information about the inserted medium, start a new Data or Audio compilation.
After that, you can open the medium information dialog by using the toolbar button or go to Disc → Disc Information (Ctrl+I) in the menu.
The following window will appear. Scroll down to see all the available fields. On the right of the window, a virtual disc is displayed, and on it used and free space, with different colors, as a pie-chart.
Notice that this dialog shows the type of the disc, if it is open (writable) and also the number of sessions written on the disc, with additional information about them, like the File System used.
A similar topic is Recoder Information.