I am planning on buying an iPhone 4 off of Craigslist tomorrow. The owner says that he had AT&T do a factory unlock on it. Is there a quick or easy way for me to tell if it's truly factory unlocked opposed to jailbroken or something else? I want to make sure that it's not going to be locked again if I need to update it or restore it. I don't know that I'll have the chance to restore it right there while we are making the deal, so I am hoping there is another way that I can tell.