To measure the effectiveness of your address fold you will have to look at the unique opened ratio of your email marketing campaign. The easiest way? Just take the number of sent emails, subtract the bounces, and finally count the number of opens on the remaining number of emails.
The success of the email’s body is measured by the actual number unique click-throughs. Look at the number of unique opens on your message and compare this with the unique click-through of the message. If the gap between unique opened and unique click-through is too high, your message was not relevant enough or the structure/programming went wrong.
Conclusion: measure the effectiveness of these email aspects regularly. Both methods will tell you exactly where your emails could benefit from further optimization.