You can visit a webpage, or use a name service lookup that you can do that will show you if your system is using Quad9. Below are instructions for confirming you are using Quad9 on your Mac.
Open https://on.quad9.net - If you're using Quad9, you'll see a "YES" answer, or "NO" if not using Quad9.
Open "Terminal" by searching for "Terminal" in Spotlight (the magnifying glass icon in the top right-hand corner of the screen).
Run the following command:
dig +short cname id.server.on.quad9.net.
If you are connected to Quad9, you will receive the following:
Note: You may receive different information before quad9.net than what is shown in the above image such as res100.ord.on.quad9.net. The important thing is that you receive the results in this format, and it ends in quad9.net
If you are not connected to Quad9, you will receive the following:
Note: When you are not connected to Quad9, terminal will begin a new line with no results shown.