Really I would tend to think in would be in the eyes, and her expression. Nipples do get hard when aroused but the also to it because it's cold, they are scared or happy, or pregnant, and simply because they can. As far as genitals I suppose some good magnate work would give the correct look for getting a clear render would be difficult. Really I think it would have to be a combination of eyes and expression. Poses wouldn't really do since the body may be posed like "Come and get some" but if the expression and eyes don't smoulder, them it looks more fake. and overdone.