Df prints out details about mounted file systems.

Example:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda6        97G   43G   49G  48% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            3.9G  8.0K  3.9G   1% /dev
...

Check out www.binarytides.com/linux-command-check-disk-partitions/ for more useful disk partition commands.