If you have access to root on *nix there is the file /etc/fstab (or vfstab), the /dev/dsk directory and using fdisk (prtvtoc in Solaris) on all the system disks you can find. On systems with Veritas, you have vxdisk list, etc. mr_mischief has suggested 'mount', which should work well.

walto's suggestion, using the module Sys::Filesystem, seems far more portable, however. Then you're not restricted to a single OS or file system.