Click the track icon at the bottom of compose box:


Before a tracked email is opened it will show a red star like this in your Sent or Inbox:


After opening it will turn green, mouse over it and see when it was opened:


Get instant notification when your email has been read:


When tracking is selected, links in emails will show how often the link has been clicked.


Note:
An email won't record as opened (read) if you're viewing the email and you sent it, the email must be sent to a different email address than your own.