I'd crack the laptop open (if you feel comfortable going so) and see what the serial number sticker looks like. If it has a printed sticker with barcodes, serial numbers and such, its the original most likely. If its blank, missing, or has a hand written serial number (or one that doesn't match the body) it was replaced. Its usually stuck on the top of RAM slots. It might be viewable by removing the RAM shield, but I'm not sure.