In WhastApp, there are two little checkboxes next to every message -- one indicates the message has been received by the server (which is actually very useful!)

In WhastApp, there are two little checkboxes next to every message -- one indicates the message has been received by the server (which is actually very useful!), two indicates the message has been delivered to the client. Coupled with the "Last seen at <this-time>" indicator for each chat partner, it's a decent heuristic for read messages.