Shoutbox

There's a trick:

Enable tabbed chat to organise all chat windows into one. Then, your contact's real display name will show as the tab's title
if at least 2 chat tabs are present (see attachment).

So the trick is: if you wanna know the real name, do as if you'll chat with at least 2 friends (you need to type nothing, of course, if you don't want to disturb them), and find their real names.