How can you tell if it's working?

Probably the easiest way to verify is to try to connect one of the WordPress mobile apps to your site (https://www.ads-software.com/mobile/). The current versions of the apps tell you that XML-RPC is disabled.

Another option is to use the XML-RPC Validator at https://xmlrpc.eritreo.it/. This was written by Danilo Ercoli on the Automattic Mobile team – blog post available here (https://daniloercoli.com/2012/05/15/wordpress-xml-rpc-endpoint-validator/).

Just plugin the URL for your site and click “Check.” If the plugin is working, the validator will return an error that the request was forbidden.