There are 2 ways in which one can Preview a template :

i) Navigate to Templates tab. Click on the Preview icon that is available in the Actions column against a template name. The preview of the template will be displayed in new tab of the browser.

ii) If you are creating or editing an email template, the preview option is available just above the body area of the template.

Tip : It is a good practice to preview the template and test whether all images, links, etc work as desired before sending it to any contact or using it an email campaign.