If I had wanted to make a character that "doesn't really look like one or the other", my initial thought would be to:
Pick a gender of male or female. Pict the appropriate shortdesc (femboy if male or tomboy if female) then mention in my character's description that their features were rather neutral, not particularly masculine or feminine.
My only concern is that I might cross a line in telling other characters how they see my character if I said something like "It is difficult to pin down their gender." Is something like this going too far as in deciding fo others how they interpret my character's features?
Also, I've never considered tomboys (for example) to be girls that plain old looked like boys. In my experiences they might simply be less interested in feminine trappings but still display the physical traits of being female.
I guess I'm just wondering if I should be having my character automatically assume that a tomboy is a man unless give reason not to when my natural impulse would is to instead assume they were female unless their description suggested otherwise. Is there a 'right answer' as far as SD is concerned?