User blog comment:IamJakuhoRaikoben/Chapter 393: Silver Memories, Review./@comment-4449950-20140717063123/@comment-7418318-20140717095507

We saw that Silver was watching the GMG, so from that alone he would already know Natsu's look, voice, kind of magic and that he belonged to the same guild as Natsu.

And from this chapter I think it's also clear that Silver did research on Gray. It'd be hard to do that without Natsu being mentioned time and again, seeing as how they are in the same team. He also knew that Juvia might be "Gray's Woman" after all.