User blog comment:Zippertrain85/The reasons why Serana is not from the First Era./@comment-121.219.169.69-20130518094845/@comment-9062114-20130518141352

There are stones in Castle Volkihar that don't appear anywhere else in Skyrim. Light collored glassy like stones. The other ancient Nordic ruins use darker stones with more irons. Also, the stonework has a very slick look to it. No Ancient Nordic ruins in Skyrim have this.

Because Vingalmo was so happy to see her. If you see his reaction it seems quite obvious that he's seen her before.