Barebones command-line utility to check if anonymous can successfully login on a given FTP server. Compile with whatever and run
ftprobe 1.1.1.1
ftprobe ftp.example.com
It will print OK\n if anon is allowed and nothing if not. Use something like GNU parallel to perform mass-scans. ftprobe puts much less strain on memory, network and CPU than lftp and even ftp. A consumer CPU should handle a few hundred parallel probes with no problems.