What is the most reliable option to see if site has been indexed?

The GSC for one of my pages shows URL is not available to Google but if I copy the page's URL and put it in quotation marks like "123.com" and enter that in a google search I am seeing the site listed in the SERP.

Does that mean the site has been indexed and if not then which tool would be the most reliable to show if a site has been indexed please?