How to make a belly look more masculine or feminine: a male's navel is often more horizontal, and a female's navel is more vertical. Males' bellies are usually flatter, have a visible Inguinal crease, and the ABS are more visible. Females' bellies tend to be more apple-shaped and have a more visible Midabdominal line.
Of course, these differences won't be true for all males and females, but knowing them lets you play around and make your character exactly what you want it to be!