To manually test an IP address

      dig +short test.openresolver.com TXT @1.2.3.4

      (replace 1.2.3.4 with the IP address or domain name of the DNS server you are testing)
If you get "open-resolver-detected" in response, then you have a problem :)

Or, use a form:

Recursive resolver is not detected on 128.199.216.206

IP address 128.199.216.206 is not vulnerable to DNS Amplification attacks.