It's likely that, as you get used to the way automations work and using templates with merge fields, you're going to to want to preview an email before it gets sent to your client. There are a couple of different ways you can go about this.
You can always use the manual compose feature to create a "one-off" email. Click "compose email" or click on an email address in the system and select an email template from the dropdown. It will run the merge process and you can see exactly what it looks like, and make any tweaks needed there in the editor before you send it off.
When you're ready to dive deeper and get a little more value out of Mago, you'll be using automation routines more. You'll simply click "preview" to get an overview of the routine actions. All of the email template names are also clickable. When you click one, you'll see the exact email that will go out, including all the merging that takes place.
A third way that also applies to emails generated via automation is the "30 minute delay." Under your profile you can turn on a 30 minute delay so that emails to go out instantly will be held in the "scheduled actions" box on the lower right of the page. You're able to preview, edit, and reschedule emails from there. After 30 minutes, they'll be sent, or you can click on an email in the routine and change the send schedule to "immediately" and it will go out right away.
We also covered this question in a webinar. Jump to 48:24