Checking the registration in the Apple database won't do you much good, because anyone who knows the serial number can register it under his own Apple ID. There is no way you can be sure that a used computer, or anything else used for that matter, isn't stolen. If you're thinking that Apple maintains a list of stolen products, it doesn't. The only exception would be a product stolen directly from Apple, for example by burglary of an Apple Store. That's a drop in the bucket of all computer thefts.