There is no 100% accurate way to tell who is online at any given time, due to privacy settings. You can see who is available for private chats, as well as the total number of people logged into the instant message chat, but it's not a reflection of who is on the site or who is in the chatrooms.
We also have "Active Now" boxes throughout the site and you can search the Member Directory, but neither of these will show Muggers who have opted to go incognito on the site.