How can I tell if my messages have been sent/delivered/read?

There are 3 icons to indicate the status of a message:

• 1) One Grey Checkmark: the message has been sent, but is not yet delivered to the recipient's device

• 2) Two Grey Checkmark: the message has been delivered to the recipient's device but not yet read 

• 3) Two Blue Checkmark: the message has been read by the recipient