You know your a multi-boxer when...

[Shadowandlight]

Moving a window or icon your desktop makes you unreasonably nervous

Also, you know you're a multiboxer when you've spent just as much time tweaking and perfecting your ISBoxer config as you have optimizing your characters' gear, stats, skills, rotations, loadouts, talents and/or AA builds.

[RSM72]

In the morning while staring at the mirror in your bathrom you ask yourself: Ok the slaves are not right behind me brushing their teeth - at what door or corner did I lose them again?

[mpfiv]

Or when you dream that the slaves of yourself aren't doing what you want them to.