This article describes how to confirm whether a message has been read by the recipient(s).
If the message has not been read, you will not see an eye icon when you hover your cursor over the sent message.
(click to enlarge)
If the message has been read and you hover your cursor over the sent message, there will be an eye icon, a badge showing the number of views, the recipient name(s), and a date and time stamp.
(click to enlarge)
Learn more about message notifications here: Message Notifications.