Are you planning to buy an iPhone from Craigslist or eBay? Here are a few ways to find out if that iPhone was stolen before you buy it. All you need is the serial number or IMEI number of the iPhone. The seller should be willing to provide this to you if you contact them.
Apple just released a tool to see an iPhone is locked down by the true owner. This method is only useful if the original owner turned on Find My iPhone and then marked the device as missing when it was lost or stolen. This is all the more reason that all iPhone users should enable Find My iPhone when they get a new iOS device.
The second option to find out if an iPhone was stolen is to call AppleCare at 1-800-275-2273 and provide the Apple support representative with the serial number. If the original owner has called AppleCare or visited an Apple Store to report their iPhone as stolen it will be flagged in Apple's internal systems and the representative will be able to let you know if the iPhone is stolen.