Look at the output of "sudo /sbin/fdisk -l" to list all of the drives and partitions.
Read the contents of the /etc/fstab file. It will contain the devices and mount points where they are mounted.
Keep in mind that the device entries may be of the form UUID=00d2b1c4-edb4-4018-944d-db6bd9e7081e. If so, look in /dev/disk/by-uuid/. ( e.g. ls -l /dev/disk/by-uuid ). These entries are created by udev and are symbolic links to a real device node such as /dev/sda1.
ls /dev/disk/by-uuid/ -l
lrwxrwxrwx 1 root root 10 2008-08-11 22:22 00d2b1c4-edb4-4018-944d-db6bd9e7081e -> ../../sda5
lrwxrwxrwx 1 root root 10 2008-08-11 22:22 145266D35266B95E -> ../../sda2
lrwxrwxrwx 1 root root 10 2008-08-11 22:22 14D23645D2362AFE -> ../../sda1
You can also boot up with a live distro or rescue mode and run fsck to check the filesystems.
If fdisk says something about the partition table being out of order, I wouldn't try to fix it. The /etc/fstab entries and grub's /boot/grub/menu.lst file use the current configuration and changing the partition table may require fixing /etc/fstab and reinstalling grub before being able to boot again. ( Yes, I did it once, and the devices in /etc/fstab where lvm2 devices. )