How do I find out what hard disks are in the system?

This is highly platform-dependent. Also different methods may treat edge cases differently (“fake” disks of various kinds, RAID volumes, …).

On modern udev installations, there are symbolic links to storage media in subdirectories of /dev/disk, that let you look up a disk or a partition by serial number (/dev/disk/by-id/), by UUID (/dev/disk/by-uuid), by filesystem label (/dev/disk/by-label/) or by hardware connectivity (/dev/disk/by-path/).

Under Linux 2.6, each disk and disk-like device has an entry in /sys/block. Under Linux since the dawn of time, disks and partitions are listed in /proc/partitions. Alternatively, you can use lshw: lshw -class disk.

Linux also provides the lsblk utility which displays a nice tree view of the storage volumes (since util-linux 2.19, not present on embedded devices with BusyBox).

If you have an fdisk or disklabel utility, it might be able to tell you what devices it's able to work on.

You will find utility names for many unix variants on the Rosetta Stone for Unix, in particular the “list hardware configuration” and “read a disk label” lines.

lsblk will list all block devices. It lends itself well to scripting:

$ lsblk -io KNAME,TYPE,SIZE,MODEL
KNAME TYPE   SIZE MODEL
sda   disk 149.1G TOSHIBA MK1637GS
sda1  part  23.3G 
sda2  part    28G 
sda3  part  93.6G 
sda4  part   4.3G 
sr0   rom   1024M CD/DVDW TS-L632M

lsblk is present in util-linux package and is thus far more universal than proposed alternatives.

How about

lshw -class disk