Testing in emulators
While developing a UWP app, you can quickly see what it would look like on various devices in the XAML design window. The following screenshot is showing the 23" Desktop (1920 x 1080) 100% scale emulator:

Switch to a phone option with a vertical layout. The following screenshot is showing the 5" Phone (1920 x 1080) 300% scale emulator. You can see that the right-hand edge is visible:

You can also run the app in a Simulator rather than on Local Machine. You can also choose Remote Machine or Device:

The Simulator can rotate, and change input modes and screen resolution, using the buttons in the toolbar, on the right-hand edge of the Simulator window:

Tip
Test your apps with the Simulator, and then test on all of the actual devices that your users will deploy your app to.