Apparently they have some abilities in the field withe respect to the dogs gender. basically you can say there are three main issues in dog's human recognition.
smell
man and woman smell differently, but not that much biologicaly , although they do use gender stereotypical smells (shampoo , types of food ), and of curse some difference in sweat.
sight
man and woman walk differently , human can see this feature (gender) in high percentages even if the human body is reduced to six moving points !
I guess there should be no reason why dogs souldn't
behavior
there is gender typical pattern of behavior , which are visible and dogs sure can peek up.
some more material on the subject ( dog's gender and human gender ) can be found at this site.
http://www.anthrozoology.org/abstract_topics/cruelty