Currently, we do not have a built-in mobile preview feature, however, you can still do it with Google Chrome's developer tool.
Here's how to preview in mobile:
1.) Open your contest in Chrome.
2.) At the top, go to View > Developer > Developer Tools
3.) Once you do that you'll be able to preview your contest in mobile.
Tip: At the top where it says "iPhone X" you can change that to any other mobile device. Hit refresh afterward to see the change.