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.
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 the recipient(s).
To see the Message info screen:
The Message info screen shows:
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.
To opt out of the Read Receipts feature, go to Settings > Account > Privacy and uncheck Read Receipts.
Note: This will not disable the Read Receipts for Group Chats or Play Receipts for Voice Messages. There is no way to turn off these features.