There are a number of different ways, but I'd always start with Google Search Console Coverage reports.
If you haven't already, sign up and configure Search Console.
Then, go to your property > Coverage.
Then select the Valid "tab". You'll then be able to click on the two types of valid ("Submitted and indexed" and "Indexed, not submitted in sitemap"). Within each of these categories, you'll be able to download a CSV of the pages lists.
In the same area you'll be able to see lists of pages that Google knows about but hasn't indexed for one reason or another.