what's the practical way to find index of website?

Depends whose site. For your own, Search Console, the Pages report under Indexing shows exactly what's indexed, what isn't and why. That's the only accurate source. For a site you don't own, type site:example.com into Google. The count it shows is a rough estimate, not a real number, and it swings around, but it's fine for a quick look at what's in there.