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
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.