on host running nagios

(assuming you're running centos or some redhat derived system)
as root
  yum install gmp gmp-devel gmp-static perl-Math-GMP -y
  use perl's cpan to install Net::SSH::Perl, Nagios::Plugin
  perl -MCPAN -e shell
  cpan> install

  edit your commands.cfg file (from RPM install, the file is
  /etc/nagios/objects/commands.cfg)
  add lines

  # 'check_FS_ssh' command definition
  define command{
          command_name    check_FS_ssh 
          command_line    /usr/bin/perl $USER1$/check_FS_ssh -H \
              $HOSTADDRESS$ "$ARG1$" "$ARG2$"
         }


   in the host definition file, add lines like:
   define service{
           use generic-service
           host_name HOST.SOME.NET
           service_description FS-c90-w80
           check_command check_FS_ssh!" -w 80 -c 90"
           notifications_enabled 1
	   }

    make sure to set the host name to the correct value

    this would check all the file systems on host.some.net with a warning
    level of 80% and critical level of 90%.
  save

  copy the perl script check_FS_ssh to plugin dir (on centos using rpms,
  the path is /usr/lib64/nagios/plugins) and set executable perms

as nagios user:

  ssh-keygen -t dsa


on system to be monitored:
as root
  create nagios user
  copy nagios@(host running nagios):~nagios/.ssh/id_dsa.pub to
     ~nagios/.ssh/authorized_keys
  make sure perms on .ssh dir are set to 700 and authorized_keys file to 600


restart nagios on your monitor host and you should now be monitoring 
file system sizes over ssh.


important caveats: these scripts rely on nagios user having a usable shell and
home directory on both the system doing the monitoring and the system being
monitored.   you'll also need a posix compliant df program availabel to the 
nagios user.   it might not be clear from the script/documentation, but the 
values you set for critical and warning apply to ALL file systems.
if you use the ignore option, it's somewhat akin to a regex.   so 
using -i var would stop monitoring (assuming they were filesystems on your
machine) /var, /var/tmp, /var/lib/mysql and /var/lib/pgsql.