User blog comment:IamJakuhoRaikoben/Chapter 375: Supernatural, Review./@comment-5303512-20140314233709/@comment-7418318-20140315101615

He could sense Zeref, yes, but from the timing of the scenes it could be that this happened at the moment when Zeref was confronted by Elfman, Evergreen and Natsu and had the two magical outbusts.

As for why he could recognize this as Zeref's power, my guess would be that there are still magical items and spells remaining from 400 years ago that carry enough of his "magical signature" to recognize Zeref himself too. That would also explain how Jellal, who never was in any kind of contract with Zeref himself, could recognize that the Eclipse left a feeling close to that of Zeref's magic.