Hostname to IP address
There should be a feature on the my.nextdns.io website to manually check the hostname to IP resolution. E.g. I want to know the IP address of "in.msn.com" I should be able to type it in a textbox and get the IP.
In the meantime keep in mind you can simply type in the URL https://dns.nextdns.io/abc123?name=example.com&type=A (replace abc123 with your config, and example.com with your URL) to test. Someone might be able to write it into a user script or something.