I have been working on flutter since couple of years. I have the low end pc and not able to test the UI for all the different sizes of the emulators and run the app on the real devices so is that any third party package available to check the responsiveness of the app like in web dev in the chrome there are multiple mobile devices to test the website UI.
Try using device_preview package
here is the link: Approximate how your Flutter app looks and performs on another device.