How do we know that?

An IP address is allocated to every device connected to the internet, allowing them to be identified and to communicate with each other. Whenever you load a website page on the internet, your device's IP is included in the request you send to the website so that the server hosting it knows where to send back the page's info. A simple look at your request gives us your IP. Note that if you're using a router, the public IP we give here is actually your router's IP, which then allocates a local IP to every device connected to it. Also, if you are using a proxy or a VPN service, its IP is shown instead of your own.