Check behind her right ear for the "artist's mark"-- there are several different ones; one looks kind of like "#e", and another looks like two y's and a little squiggle. They're not so deeply incised as the Pleasant Company mark, and once in a while the hair can cover part of it.
Also, the older Felicity has fuller cheeks, redder hair, peachier skintone, and a slightly thicker body. Try checking her against the pictures of the newer Felicity on the americangirl.com site.
Hope this helps!