Right click them or something. So you right click them and it shows who they are and where they're online, right? If they're not moving, right clicking the player also tells if they're afk or not.
~ @Toostenheimer
I think something like WoW's AFK system works best. After x minutes completely not doing anything, including commands, then you become AFK until you start doing things. Then, whoever messages you receives a note saying you're afk.
This isn't part of it, but then, when they're back, it gives them a notification.