How can see the SNI?

In Firefox Bash Script Third Party

When you connect to an server that supports SNI, your browser sends a servername, and the load balancer uses that to select which server it should serve. If a client doesn't support SNI, the load balancer will serve the SSL/TLS certificate you set as the default. These sites use * for their default.

To verify that is happening you can check the lock icon at the top of your browser, if it's green then the browser is happy that the certifcate is valid, trusted, and configured correctly. For most people, this is as far as SSL/TLS validation goes. To see the SNI you need to look a little deeper. The links above will show you how to do this in Firefox (Chrome removed the easy version of the same thing recently), with a bash script using OpenSSL, and links to a third party SSL/TLS validation service.