I take it you're in some windowed environment like KDE.
You can either open the program memnu and open a console from there, or you can hold Ctrl and Alt and press F1 (or F2 ... F6). The first will open a command line window, the other option will send you to a full screen version of the same thing.; there you will also have to log in new.
Whichever option you use, you will be at a command line now, and you can simply type df then Enter. You'll see the list of your partitions.
After that you can either type exit or close the window you opened, or if you switched to a full-screen console you can return to the GUI by holding Ctrl + Alt + F7.