At any time during the creation of a Mosaico template you can click the Test button at the top right of the page to check what the recipient will actually see. There are several options as part of the Preview/Test screen:
Preview
As HTML will render the mailing in a full HTML format.
As Plain text shows what a recipient will see if they don't have an HTML email client (very unusual these days!)
Send test allows you to actually send an email from your system:
To an individual (defaults to your email address, but you can change)
To a test group
This is an example of a HTML preview of a mailing:
This is what a plain text test email might look like:
Once you are happy with the template (you can always go back and amend it), you can click Close to go to configure when the mailing will be sent, and other options. This is covered in the next article.