Determine whether Logic Board has actually been replaced
You can compare the serial number on the bottom of the laptop casing to the serial number shown in System Report by going to Apple Menu -> About This Mac. When I worked at Apple, it was known that replacement logic boards would carry the new SN (naturally) and that cases from the old SN would link to the new one. 

The serial on the laptop itself is the original SN it shipped with (unless the bottom case has also been replaced) and can be found in very small print near the display hinge. On the Mac, check for an SN that diverges or is not present, as that indicates repair.