Recently WhatsApp updated its mobile App for platforms like iOS, Android and others with some features that let you know if the message you had send to someone has been read or not, knowing it is pretty simple.
The Single Grey Check mark
When you send a Message to someone if you see a single grey check/tick mark against it then it means that this message is being send to WhatsApp server. The message is yet to be delivered to the person whom it was intended.
The Double Grey Check mark
It means that the Message has be delivered to the person who it was sent but the person has not read it yet!
The Double Grey Check mark turns to BLUE
Now when the double grey tick mark turns blue it means the message has been read by the person.