Email Templates can be A/B tested against multiple subject lines.
After a template is tested or is sent through an email campaign, the performance of these can be checked in 2 ways.

i) Navigate to Templates tab.
Against the name of the template, a variant icon appears next to the text Multiple Subject Line. Clicking the variant icon will display all the subjects and the counts for sent email, opened email, clicked link and browsed page activities for each of the subjects.

ii) Navigate to Templates tab.
Click on the name of the template. You will be redirected to the detail page of the template. A complete statistics is displayed for the given template alongwith the counts for individual subject names.