Normally, if you try to view a deleted message or a message from a private room, you will not be able to view its content. Deleted messages show as "(deleted)" in the room itself, are absent from the transcript (
http://chat.stackexchange.com/transcript/message/ + ID of message) and give a 404 if you try to view history. Ordinary users cannot access private rooms directly or through the transcript, and history for those messages gives a 404.
But if you know the ID of the message you want to view (and this is guessable especially on low-traffic chat sites like meta.stackoverflow), you can view the contents, needing only 20 rep to be able to post a chat message. To do it, post a chat message
http://chat.stackexchange.com/transcript/message/+ ID of message
on any chat site, and the message will be oneboxed.
For example, from chat.meta.stackoverflow.com (where I am not a moderator), I can view the contents of a deleted message by entering
I can even view a message from the super-secret moderator-only Teachers' Lounge room by writing
The message contents are visible from when logged-out, by browsing to the chat room. See http://chat.meta.stackoverflow.com/rooms/38/conversation/exploit.