When you use the karma email integration, a tracking cookie is embedded into the email and will notify you when the email is opened by the contact.
In your header bar, there is an icon that looks like a bell, this is where notifications are housed. If you are offline when the email is read, you'll get notifications. To read all of the notifications, click on the bell icon. See All will allow you to see all of them instead of just a few.
If you are online when someone reads the email, you'll notice a black box in the upper left corner that will display what the email is and the contact's name, and the fact that they opened the email.
Within the history of the contact, you will also be able to see a timestamped read receipt of the email.