So with that introduction here's a post on the electrocardiographic diagnosis of posterior MI from one of my favorite emergency medicine blogs.
Posterior MI has been the victim of confusing terminology. It often occurs along with inferior infarction and decades ago inferior and posterior MIs were lumped together such that all inferiors were termed “posterior” by many clinicians. Later, in an attempt to make the proper distinction, posterior MIs were designated “true posterior.” The traditional method for easy electrocardiographic diagnosis is the mirror test in which you flip the tracing over and upside down, hold it up to a light and “read through” the back side, premised on the idea that leads V1-3 provided a mirror image of the indicative changes.
The above referenced post goes through the diagnostic criteria as well as the placement for leads V7-9. By the way, V6 is right next door to V7. Don't go to the bank on it but on the standard 12 lead it may offer some clues.
Image: Mt. Moran, Grand Teton National Park, and its mirror image in Jackson Lake.