Well, to know what filesystems (not specifically hard drives) are mounted, just type 'mount'. There you will see what is the mount point (usually /) and so on. If you tipe umount -a, you will unmount all the filesystems, except those who are being used (if you are in a directory of a mounted filesystem or running an application or looking a file from there you cannot unmount it).
Once unmounted, you can do a fsck /dev/hdx, where x is the number of the hard drive partition you want to check. Also, 'man fsck' is useful.