Stock birfs will have "birfield ntn" stamped on the outside of the bell along with some other numbers. Chromo longs have "longfield" etched on the flat part of the bell.  I have bellfields and they are stock birfs that have been treated and a ring welded on the bell. They have "BF" stamped on the end of the stub shaft. Not sure about other brands, but I think longfield and marlin's are the only "chromoly" birfs and they look totally different than stock birfs.