I've run these kinds of tests before, so this isn't my first time around. To really test my ad blocker, I like to go to sites that I know are loaded with ads. My process is pretty simple, I'll turn off my ad blocker, find those sites, and then turn it back on. Once I clear my history, I'll go back to those sites and see how it performs. Another thing you can do is use other extensions to see how many ads are being blocked in real time.
Let's say you're using a DNS filter. A great way to test it is to pair it with a browser extension like uBlock Origin Lite. The process is pretty simple: disable both your DNS filter and uBlock Origin Lite and find a site. Then, turn on just uBlock Origin Lite to see how many ads it blocks. After that, go ahead and enable your DNS filter and refresh the page to see the difference.
It's a practical, real-world test, instead of relying on a theoretical test, you're using real websites that have a variety of ad types, which provides a more accurate assessment of your blocking setup.