There is 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 Linux.
Open Terminal. Instructions for opening Terminal vary based on your Linuxso instructions for opening Terminal are being omitted here.
Run the following command:
dig +short txt id.server.on.quad9.net
If you are connected to Quad9, you will see output similar to 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 get a failure like this:
Note: When you are not connected to Quad9, terminal will begin a new line with no results shown.