Um...did you miss the Login/Logout button? It says Logout if you are logged in or Login if you are logged out.
Of course there is also the small clue as to whether or not you are logged in on the titlebar of Internet Explorer for every Channel 9 page you visit, such as "Am I logged in? (Logged in as: Nystul)".
The login/logout button does hint you someone has logged in. That person doesn't necessarily have to be you.
However the target audience of Channel9 are developers that are used to deal with logic like 'if you're able to login then you're logged out', you shouldn't rely on logic. Some things just have to be shown on the spot. If it's pure logic that rules the world
you shouldn't need to comment your code (since the code says it all.)
Another point that hasn't been pointed out yet: if you're logged in you should be on the 'Online Users' list. (I haven't really checked if it's true in every case.)
As for the titlebar comment, the title bar doesn't tell you you are logged in as
<username> on every page. The homepage is a good example. Besides that, the titlebar is a perfect spot to miss.