@RoyalPheonix , like
@ed1960 said, if they're not in your fellowship, you can't. The game simply doesn't allow that. The best you can do is send mail to them and hope they'll reply, though doing that just to see if they're there is suboptimal at best and can result in complaints and official action at worst.
On the forums, if you want to know if the guy you're replying to is online, the fastest way while in a thread is to check if a green icon is below their avatar, as shown here:
View attachment 12970
Hovering over it reveals the tooltip shown above, as well. Too bad the screenshot utility doesn't also capture the mouse pointer
, but ah well. what can ya do about that?