User blog comment:Miskos3/Fairy Tail Chapter 472: Laxus vs. Wahl, Review/@comment-24102956-20160210232211/@comment-1776824-20160210232709

Mages have been shown to sense magic power, so they can just feel him sitting on the ship, doing nothing. .-.

In Laxus' case, they had technology that let's them directly trace it. Wouldn't be ideally difficult to match the signatures and see where he was once they found out he was fake.