Hey @Thomas, you can use the check_ssh plugin for monitoring ssh services. 
The commands.cfg file contains a command definition for using the check_ssh plugin, which looks like this:
define command {
    command_name    check_ssh
    command_line    $USER1$/check_ssh $ARG1$ $HOSTADDRESS$
}
A simple service definition for monitoring the SSH server on remotehost would look like this:
define service {
    use                 generic-service     ; Inherit default values from a template
    host_name           remotehost
    service_description SSH
    check_command       check_ssh
}