Unfortunately, Dorflinger cut glass is not marked.  You have to rely purely on shapes, sizes, colors, and years of experience in order to correctly identify a piece.  If you are really lucky and you are dealing with Dorflinger glass that also has silver as part of it, then the silver could be marked as Dorflinger.  However, if you encounter glass that has any marks suggesting that it is Dorflinger, then I would be very suspicious of it.  Virtually no glass from the American brilliant cut period will be marked.  The designs and quality are what identify the maker.

Dorflinger Sterling 120 Mark