I never really thought about it until I started setting up my first team, but I believe that I ran into a 5 man sPriest group in a BG once. I didn't notice their names, but I do remember that they were all in the same guild and were absolutely WRECKING my mage (and the other players in WSG at the time) the just DoTd and ran away... and I was dead.

But I just thought it was a premade, so never even put together that it was a multibox team. I think unless you see one in a city, there is really no way to tell easily, as the odds of running into them in the world is small on most servers. I really don't know how many multiboxers are on my server. but I hope there are more!