Re: How to Check if a Message is Unread?

There was a discussion  on this topic in May 2013:

http://jabber.996255.n3.nabble.com/Message-Read-Receipts-td31029.html

And then there's XEP-0333: Chat Markers (created also in May 2013), therefore might be a result of the discussion. Itt covers read receipts.

But since the specification is quite fresh and still in experimental state, I guess, there aren't (m)any clients which implement it. (I don't know any, maybe Facebook does, because they use XMPP and have read receipts).