Currently one must go to the members profile to see if (s)he is on or offline. Perhaps it would be useful if when you do a mouse-over the member name on the forum page (currently displays 'View the profile of ..) it were yo show if that individual was online or offline.
I think the little box on the left side of the user name indicates whether they're online:
Just curious, what is the use of knowing whether someone is online?
Interesting. I had not noticed that little square.
In another Thread I found someone with a light green square and when I moved the mouse over the square it said "online". And when I move the mouse over grey squares it says "offline"
I imagine I am online while typing this so why is my square grey? 
...R