It's a Federal stone if it's rounded at the top. If it comes to a point, it's a Confederate stone, irrespective of the Emblem of Belief inscribed.

I believe, that even Confederate tombstones may be inscribed with an emblem other than the Southern Cross of Honor, if so ordered by the
next of kin.

Of interest, see:

http://www.imdb.com/video/cbs/vi826318105/

http://www.cem.va.gov/hm/hmemb.asp