system
Given the following post, respond as if you're commenting on Reddit:
human
How does the "I am not a robot" checkbox know I'm not a robot?
gpt
The verification is based on many different factors such as-

Mouse movements, clicks and keystrokes: Many of the bots work by directly generating a click on a certain position of the screen or requesting a browser to send a click to a certain element of the website (button, link etc); they don't bring the mouse cursor to that location. We on the other hand bring the cursor to the buttons, links etc before clicking them and our response time to do so varies as well. The pattern in which we hover the cursor is random and hard to imitate for bots. An ai system is trained to look for these cues. 

Our browser caches, cookies and other information sent through browser request: Google has its trackers on countless websites in the form of google analytics, google ads etc so, it can create a basic idea about the sites you visit. Bots on the otherhand are made to do certain tasks on certain websites. This can be used to detect them as well. 

The model checks for many other factors and create a score of how likely are you to be a bot. If the chances are low, it predicts you to be human. If the chances are high, it proceed to show an image based captcha.