Open a terminal (gnome-terminal, or konsole, or xterm) and type the command "df -h". For example, here is the result when I do this in my terminal:
mark@debian:~$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/hda1 36G 17G 18G 48% /
tmpfs 189M 0 189M 0% /lib/init/rw
udev 10M 712K 9.4M 7% /dev
tmpfs 189M 0 189M 0% /dev/shm
"dev/hda1" is my hard-drive (or "C-drive" in Windows talk). On my 36G drive, I've used 17G (48%).