Going by my experience with the Invision forums that had that feature enabled, it was possible with that type of forum software. But even then it was only accurate to within 5-10 minutes. By the time someone showed up on the list (assuming they hadn't set their preferences to stealth mode), they could already be somewhere else.
I don't know if it would be possible with XenForo. I've never really paid much attention to the Who's Online list much, so I never noticed.
If you want to know if a specific member is somewhere, you can take a look at their profile page. If they're not in stealth mode, their whereabouts should be visible to you.