Imessage delivery status
If the message was delivered, whenever the recipient reads it, a Read [Date] will automatically be displayed beneath. Bear in mind, as Emil points out in the comments, that for this behaviour to happen, the receiver must have the Send Read Receipts option enabled. 

If the message is marked as not delivered, it means that, for some reason, that message couldn't be sent. Although you solve the issue, it wont be automatically sent, therefore requiring you to tap on the exclamation mark and choosing Try Again.