How do I test if MySQL "require secure transport" is working? I activated the checkbox named "require secure transport" in the options file, found in the MySQL Workbench.

How do I know if MySQL "require secure transport" is working? Should I use command line at this point?
Just try to connect in a non-secure way and be sure it doesn't work.

If you are not sure wether you are using SSL or not to connect, then yes, the simplest way is using the command line client:

mysql --ssl-mode=DISABLED