How to use imei to determine if iphone is stolen or not?

Planning to buy iphone 4 from a craigslist seller i was told to use the imei lookup to see if its stolen or not how do i use this?

If your located in canada http://www.protectyourdata.ca/check-the-status-of-your-device-in-canada/ can check it for you. http://swappa.com/esn also can do the checks for you.