For iOS, the log output is displayed in the Xcode organizer: click on the Console under your device in the left side panel.
For Android, if you launched the application on the device using forge run android, the output will be transferred to your terminal. If you have not started the application from our toolkit, the log output is still available: run adb logcatin the terminal.
In both cases, make sure that you have the registration module configured correctly. If you have a level set ERROR, debug level messages will not be displayed!