Cisco ISE (Identity Services Engine) learns information about devices through multiple methods. First, traffic generated by the device can give insights into the device's behavior and identity. Second, RADIUS (Remote Authentication Dial-In User Service) attributes provide essential authentication and authorization details that help identify the device's user or role. Lastly, user authentication to the ISE helps in establishing the identity of the user associated with the device, thereby providing critical information. These three methods are essential for Cisco ISE to gather comprehensive data about connected devices.