Related page: canUser
This answer only applies to modules rendered within the UUX / Online Banking application.
To determine whether or not a user is in the mobile app, you can check the window.Tecton.inMobileApp property. It will return true if you are in the mobile app and false if you are not. This property is determined by the platform when your module calls the connect method.
Alternatively, if you would like to know if the user is on a mobile device (e.g., in the mobile browser vs on the mobile app), you can check the window.Tecton.device property. This returns a value of phone, tablet, or desktop depending on the platform's determination of the user's device.