As soon as you're in the content editor of your mail campaign, you'll be able to preview your mails the way they will look like in your recipients email applications. Click the 'Preview campaign' drop down menu on the right to open up the options.
'Enter preview mode' opens a new tab with the full email preview.
The option 'send a test email' opens a modal where you can enter one or multiple email addresses to send a test email to. If you enter multiple email addresses, make sure they are separated by commas.
Click 'Send test email' to actually send the emails and go back to the editor. The test email will show
[TEST] in the subject line.