if buying from ebay a macbook pro ,which is listed sightly used...after i have bought the laptops ...is there a way to know if some thing was changed inside from what came in it originally from apple? i mean a cheaper hard drive or a different airport card , ram etc by opening it up? are there some unique numbers on each component to figure out if it belongs to that system? (other then the serial outside) ...what are the things i should look for?