You should be using the built in email render testing in Account Engagement (F/K/A Pardot) to check the appearance of your emails before sending every single time. This feature is, unfortunately, only available in the higher package levels. If you have it, you should be using it. This is because every time you write a new email, you have fresh opportunity to accidentally mess something up (it's ok, we've all done it).
The built-in Litmus based tool is great for checking the visual appearance of an email, but it's not as good or useful at checking all the other little things that could be off. Enter Mail-Tester.com as a free second quality check, that is available for any edition of Account Engagement (Pardot).
This tool will check all the technical, under-the-hood bits of your email and tell you what looks good and what might need attention, and also checks it for "spammyness" in its content. Here's how I recommend you use it.
Step 1: Visit Mail-Tester.com you will see an email address there. It renders a new one every time you load the page. Copy this address, it's going to become your permanent address you use to test your emails.
Step 2: Create a Pardot/Account Engagement prospect using that email address. You might want to make sure this prospect is in your global suppression list and blocked from syncing with your CRM.
Step 3: Create an Email Test List for your Mail-Tester prospect, and add that prospect to the list. There's a checkbox when creating a list for email test lists. This type of list sends as it if the send were a real send, not a test send. This is essential for getting a good real world check of what your email will be like when sent.
Step 4: Use it! When building your email, use the testing step and send to your test email list.
This is what that looks like in the Lightning email builder (top) and the classic email builder (bottom).
Step 5: Wait a couple of minutes. Take a break! Look out the window. Stand up and stretch.
Step 6: Head over to Mail-Tester.com and paste the email address you used to make your prospect into the box (instead of the one that it auto-generates) and click check your score. You'll get a comprehensive score on the email that supplements what you will see in the built-in Litmus testing.
Step 7: Repeat as needed. I recommend every time you build a new email. You can re-use that same email address for testing over and over again.