Visit http://entnemdept.ufl.edu/nguyen/morph/khuong1.htm to know more about Dr. Khuong B. Nguyen, expert in Entomopathogenic Nematode Systematics and Taxonomy from University of Florida, and to get several links for his pages containing classification keys and illustrated descriptions of Steinernema and Heterorhabditis species.
You have to conduct both molecular (D2-D3, ITS) and morphometric methods to identify entomopathogenic nematodes. There is a great number of publication belong to Patricia Stock, Nguyen, Vladimir Puza, Byron Adams and Harun Cimen.