Websites can know that a user is using a VPN by checking the user's IP address. VPNs assign a different virtual IP address to the user, distinct from the user's IP address. This virtual IP address is often shared with other VPN users and may be located in a different location than the users. Websites can detect the use of a VPN by comparing the user's IP address to a list of known VPN IP addresses or by using geolocation services to determine the user's actual physical location based on their IP address.