That is propriatary and therefore kept a secret. There are a number of different checks they do in the backend. Things like getting the exact browser, operating system, configuration and even things like the size of the browser window. They also collect timings and movement of the mouse cursor. And also cookies for social media sites to associate you with your online persona. All of this information is sent back to the operator of the robot check, most commonly google, where they presumably are looking for known fingerprints of robots. If they find something suspecious they will send you additional challenges that is harder then just clicking a button.