When you Preview a newsletter, there are tabs across the top to see what it will look like on a desktop, mobile and tablet device.
To preview a newsletter:
Open the newsletter it in the Author
Click the Preview button in the top right corner
The first 3 icons in the top menu bar are for previewing the newsletter on a Desktop, Tablet and Mobile device. You can toggle between these devices by clicking these icons
Alternatively, you can send a proof version to yourself or someone else to view the newsletter on a real mobile device.