Surprisingly and without the need for an app update, the WhatsApp Messenger he started showing users new blue tics indicating when people actually read his messages.
Check the caption, according to the application's FAQ:
The cool thing is that you can also touch / hold any message and access the "Info" option in the menu popover to see more details:
The screen shows the exact delivery and reading times for each message:
Within groups, the two ticks only appear when all contacts have read the message.
If you still don't see the feature, be a little patient as it is being released little by little to all users. It remains to be seen, too, if WhatsApp allows the function to be disabled as well as possible with the indication of the last time you opened the app.
(required to all who sent us this tip!)