Others mention the mouse motion, and monitoring your other traffic to similar sites. When it shows the checkbox, it has already determined you are probably human. If you had suspicious activity, they will give you more advanced tests instead of just a checkbox.
It’s also checking your other traffic. (Since Cloudflare handles traffic for so many companies.) Are you visiting other sites in a realistic fashion, or are you doing 99% of your traffic trying to do one thing over and over.