How to know when the person has read the message.
In addition of knowing if someone has read a message, you can know at what exact time he´s done it. Whatsapp has a hidden screen that shows information relative to the reception time and the reading time of each of the messages. This only works if the recipient does not have a “Confirm Read” mark in the privacy preferences of their account.
This option also shows who in a group has read and received a message.
How to do:
Go to a chat screen and hold your finger by pressing a few seconds on one of your sent messages. Then up to the right, click on the 3 vertical dots and an information info will appear in the menu, in the case of Android, or in a pop-up menu with iOS. Click to access a screen with the details of reception time and reading time of that message.