You can preview the template from within the template library by hovering over a template and clicking on Preview. This will open a preview of the template in your browser.
While creating or editing a template from the template library or within a campaign setup, you can preview the email using the blue preview button or the "eye" icon. You can also change the device view so that you can see how your email looks on desktop, tablet or mobile devices. CAUTION! From the preview screen, be sure to use the close preview icon or click on Edit Mode instead of the Back button. The Back button will take you all the way back to the library or template selection step without saving your changes.