When you are creating or editing a campaign you can easily preview how the email for the campaign will be displayed in desktop, tablet, and smartphone email programs.
When you are editing the email itself, there is a Preview button in the upper-right of the editor:
On the CAMPAIGN DETAIL page, there is a Preview button in the upper-right of the page:
Clicking either Preview button displays the email for the campaign in a preview window.
You can click any of the three tabs in the preview window. Each displays the email as it will appear in a typical email program on a desktop, tablet, or smartphone.