How to check installed OS X version from the command-line on each partition?
The following command will give you the OS X version installed on  volumeName:

defaults read /Volumes/volumeName/System/Library/CoreServices/SystemVersion ProductVersion


To get a list of each volume's OS version, you need a simple script that will get a list of volumes, search each one for the SystemVersion.plist, and report any that are found.

I found a script on on Mac OS X Hints, by Hal Itosis, that does just that (reposted below for convenience).

#!/bin/bash -
PATH=/bin:/usr/bin
export PATH
IFS=$'\n'

vList=$(df -l | sed '1d;s:^.*%[[:space:]]*::')
sPath=$(find -x /Volumes -maxdepth 1 -type l -not -inum 2)
pFile=/System/Library/CoreServices/SystemVersion

SysVers ()
{
   if [ -e "${1}${pFile}.plist" ]; then
      n="$1"; if [ "$1" = '/' ]; then n="$sPath"; fi
      x=`defaults read ${1}${pFile} ProductVersion`
      b=`defaults read ${1}${pFile} ProductBuildVersion`
      printf '%-16s %-8s %-8s\n' "${n##/*/}" "$x" "($b)"
   fi
}

if [ $# -eq 0 ]; then for v in $vList; do SysVers "$v"; done
else while [ $# -gt 0 ]; do SysVers "$1"; shift; done; fi
exit