Ideally, a reader should be able to tell just by looking at the face if a character is male or female. In this section, I'll give some tips as to how you can draw your characters so this distinction can easily be made. I'll discuss a few critical facial characteristics, and how they can be masculine or feminine. Keep in mind that a male character doesn't have to have all masculine features, and vice versa. It depends heavily on how you want them to look.
Eyes
Eyes are very important for expressing emotion. I don't find, however, that eye shape has a lot of bearing on masculinity/femininity. For girls, you'll want to use a darker outline, or actually draw eyelashes. The eyebrows are a little more tricky. For men, you'll want a thicker eyebrow, set close to the eye. A feminine eyebrow, at least in the idealized sense, is thin and arches well above the eye. Also, the shape can be important. Eyebrows that are thickest toward the center of the face usually look more feminine, while greater thickness toward the ear looks more masculine.
Mouth
The only way to affect the mouth in the context of this discussion is with lips. In black and white lineart, you can't use too many lines, generally, or your character will look all craggly. Generally, I don't even draw lips, except for some characters. If I do, I only draw a single line under the mouth line. For women, it will curve up gradually at the ends, for men it will be straight, and curve down at the ends.
Shape of the Head
The principles that hold here are true for applying masculinity or femininity in every line you draw on a character. Strong, straighter lines for males, thinner, curvy lines for women. Men should have more corners than women, generally. You should of course play around, and see what works for you. Try some of these principles out, and see if they are compatible with your style.
