You can check exactly which fraction of the results were fetched from Brave's index vs. third-parties using the "independence score" found in the setting drawers (opening can be done with the cog icon at the top right of any page on search.brave.com). There is there a global and personalized score of independence (respectively aggregated on all user's and for your queries only).

Explanation is also found here with screenshots: https://search.brave.com/help/independence