How To Know When Your Messages Have Been Read On WhatsApp

You can now know if your contacts have read your WhatsApp messages. This does not require any expertise. This is how it works; if you see two blue check-marks next to your sent message, then the recipient has read your message. In a group chat or broadcast message, the check marks will turn blue when every participant has read your message.
 
You do not need to update the app, as this is something that the WhatsApp team has done on the server side.

If you do not see two blue check-marks next to your sent message, the recipient may not have opened your conversation, recipient may be using outdated version of WhatsApp,or it could be due to internet connectivity issue.

Message Info
For any message that you send, you will be able to see a Message Info screen, displaying the details of when your message was delivered, read or played by your recipients. .
To see the Message Info screen, Open a chat with a contact or a group, Tap and hold on your sent message, Then Tap "i"

When the message info shows Delivered, it means that it has been received by the recipient but not yet seen.
It shows Read or Seen when the recipient has read your message or seen your picture, audio file, or video. For a Voice Message, the recipient has seen but has not yet played it.
When the recipient has played your Voice Message it shows Played.

Please note that when a participant leaves a group, the Message Info screen will still show the original information with all participants, including the participant who left the group.

 source