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