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
You can use a query on the server to get the variable require_secure_transport
:
SHOW VARIABLES LIKE 'require_secure_transport';
Once your connection is open you can look in the session info view, in the lower left corner to get your connection details, including the SSL status.
For a bit more info use the Server Status
section in the management/administration part of your SQL IDE session. This text is at the lower end of the status page.