Well with the PowerBook, it comes new wrapped in a sealed foamish material that you can definitely tell if it's been taken off and then put back on. Also, when you bootup your PowerBook for the first time you're prompted to go through a registration process to input your name and information to register your new Mac. If this prompting screen doesn't appear upon bootup, then it's already been booted up before and therefore it's technically not new. That alone is the biggest giveaway.