There are three ways to preview/check your designs:

1. Play mode: Click the Play button in the App bar or press Ctrl/Cmd + P. This dims the sidebars and hover effects etc. can be seen. Press Esc to exit play mode.

2. Open in browser button: This opens the page in a browser tab/window and reflect changes in builder automatically.

3. Ctrl/Cmd + .: This keeps you in the editing mode and hides the sidebars