Index -> Interface -> Casual Games Lobby -> Observing Games and Getting Info on Games
You can click the question mark button on any Game Medallion for information on the game's setup and participants.
Any game with an "eye" button on its Game Medallion allows observers. Clicking that button will take you into the game as an observer. You are able to chat with the players and watch the game unfold, but you will not see any planning phase actions, hand contents, or other secret information. You may stop observing a game at any time by right-clicking on the map (or ctrl-clicking, if you are using a Macintosh) and selecting Return to the Lobby.