There should be an area at the bottom of the Community page that says who is online, or perhaps something next to the name of the user when he posts that tells whether he is online or offline?
We used to have that, but it caused a lot of lag. We also have the offline/online button next to the name on someone's profile which is supposed to be green when they are online, but it doesn't work. It's been suggested a lot and either the community will implement something like it soon, or they'll just decide not to
