Few things you could actually look for. Assuming it's a unibody MacBook, the serial number should be printed somewhere (check around the RAM slots). I'm sure this is applicable to older MacBook's but I just don't know where to look. Second, check the serial number once you're in OS X through "About This Mac". This won't necessarily prove anything as the scammer, if clever, could've had a logic board with a blank serial number and then flashed it with yours but I doubt it because a few things would have had to happen. So your best bet is to check the logic board for your serial number - that's something they couldn't change, IIRC, it's not a sticker of your serial number, it's actually printed.