You should try the gallery app: https://play.google.com/store/apps/details?id=io.flutter.demo.gallery&hl=en In there, when you press the menu button you can set platform mechanics to be Mountain View (Android) or Cupertino (iOS). That way you can see how it’s going to look in either platform.
Here is an example of selecting Cupertino or Material with code depending on the platform: https://medium.com/flutter-io/do-flutter-apps-dream-of-platform-aware-widgets-7d7ed7b4624d As far as I know you have to choose the widgets like this.