You Mac users probably know about this already, but in case you didn't, go here: http://www.macheist.com/ There's a bunch of software that they normally charge for, only it's freeeeeee for the next few days. No idea what's there - I'm not a Mac guy - but hey, maybe you are. Enjoy!
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Your router software should have a log that will show you devices currently active or maybe even historically using your network, just so you can confirm. It may list IP address, or perhaps even device names.