I've got the $150 off student coupon for MacBooks at Best Buy, which expires in a couple of weeks. I'm looking at getting an MBA, unless a great deal on a 13" rMBP comes up. Here's my question: Before buying & walking out with the computer, I want to make sure it is in good shape. I've read about how to run a diagnostics, which I would do. What else should I look for or check out to make sure that I don't have a lemon, or that the original buyer didn't somehow mess up the computer before returning it? Any advice or help is much appreciated - thanks.