In the Editor view you can preview an image before and after it's been processed, including the file size, type, and file name. All of these changes are visible in realtime. No need to process or export.
You can go back to the Grid view, by pressing the "grid view" button in the toolbar. The "Return" key will also enter and exit the Grid/Editor view.
The defualt view for when Squash launches can be set in the Preferences.