Fake Dupont serial numbers, specifically ones for the S.T. Dupont Ligne 2 lighter, are sometimes placed in a straight line, according to eBay. Authentic S.T. Dupont Ligne 2 lighters have the numbers stamped slightly out of alignment. In addition, Dupont lighters have serial numbers that are stamped rather than engraved.
Some fake Dupont serial numbers start with the sequence "4FK," which is a sequence that genuine Dupont serial numbers never use, explains eBay. The serial number appears on the bottom of the lighter, so if an ad selling the lighter doesn't show the bottom, the lighter in question is probably a fake. Legitimate lighters also have a dot of yellow paint just under the refill valve cap.