User blog comment:IamJakuhoRaikoben/Chapter 402: Iron Fist of the Fire Dragon, Review./@comment-83.255.195.39-20140929220624

It looked to me like Marde geer maybe knows who Natsu is because after Natsu punched him he was smilling like he was happy about something. Maybe Natsu avoiding his thorn attack so easily and then damaging him with the punch confirmed for Marde Geer that Natsu is who he thinks he is.