How to enable read receipts for messages in iMessage on Mountain Lion?
From the Messages app section of the Apple Mountain Lion "what's new" page:


  Turn on read receipts, and they’ll see when you’ve read a message.


According to this OS X Daily post, you can send read receipts in the Message app by following these steps:


  
  Open Preferences in iMessage, and select the “Accounts” tab 
  Select
  the Message account from the left 
  Check the box next to “Send Read
      Receipts” to enable
  Close out of iMessage Preferences
  


Screenshot from this post:


I can confirm the presence of the checkbox on my own Mac.