Creating a campaign and include its tag in your email. You can then see which campaign is performing best by viewing the data grouped by tag.
How do I A/B test emails?
How do I A/B test emails?
Other Helpful Resources
-
Mail Tester
Spam-check your emails before they send -
Postbin
Debug webhooks easily -
Google Postmaster Tools
Check Google Sending Reputation -
MXToolbox
Troubleshoot DNS issues