If the Glock is all original there should be three sets of serial numbers. Barrel, Slide and Frame all match. If any of those are missing or different you might want to think twice on the purchase unless you know you are buying a modified handgun. As far as checking the pistol's status, (lost, stolen, etc.), you would need to have a law enforcement officer run the serial number in the NCIC system, or have your friendly neighborhood ATF Agent run it for you.
Also this is NOT MY PICTURE I DO NOT OWN ANY RIGHTS TO IT. IT COMES FROM THE OPEN SOURCE WEB.