Frank wrote:

Cebik's book on NEC modelling, at www.nittany-scientific.com, for $50 is
well worth the money.

Or try 4nec2 at http://home.ict.nl/~arivoors/. It has built-in
geometry- and segment-checks which will deliver instant error and
warning reports. Furthermore it's completely free to use
and will run Nec2 (and Nec4, if you have a Nec4 license).

Arie.