(1) One has to remember to check "Request read receipt" before sending the message.
(2) If one forgets to check that box, then there is no way in which he can know if the message was read. (If the targeted user doesn't reply, of course)
(3) There seems to be a way to bypass this :
When you receive a message with a read-receipt request, you may have the option to read the message while denying the read-receipt request. To do this, simply click the 'Deny Receipt' link rather than the title of the message, if it appears.