[Striiker]

The worst offenders are the ones that actually group with others when obviously boxing. I had this happen in Blackburrow long ago. Someone and his wife were playing. She rarely said anything, could not follow us to save her life so we would all have to stop and wait while "she" caught up with us (particularly when we'd drop down to the ramp from the top level etc. I submitted a report on the petition forum but I know this "team" is still playing on the servers. I ran into them in Upper Guk later and she couldn't follow us to the scryer properly (because that path is a bitch to navigate on one computer as it is). He'd always make an excuse like oh, she's new here, or having computer issues etc.
I've been in other instances where I suspect a two-boxer.. USually because one of the characters do a crap job healing, fighting, etc. This is particularly evident when a situation changes mid-fight.. Often someone comes close to dying while the melee switches target and the healer stands there doing nothing or the healer is healing etc and the melee is standing there doing nothing because the mob moved. I've seen this with bad players as well but sometimes it gets very suspicious.

On a side-note, I'd love to learn how the GM's verify their suspicions. I have always assumed it involved getting the two players to do something simultaneously etc. with no warning (like both type something.. both do some movements..)