Some things are obvious...external cosmetic defects, cracked screen, doesn't power up properly, screen colors way off, et cetera. But, assuming one is at the Apple store and in the process of buying a new iPad, is there a list of what to check to make sure the unit you're buying is at least superficially problem free? As an example, if it has a bad pixel, what's the best screen that might already be on the iPad so that you can really see it? Is there a list of these kinds of hints? Thanks!