Article in English: Female goldfish tend to have a rounder, thicker body shape than males of the same age and species.  They also tend to be deeper-bodied, which is useful for identifying sex from a side view. As breeding season approaches, the females will begin to develop roe (eggs), which may cause a bulge on one side, making the female look asymmetrical or lop-sided. The vent (anal opening) of a female goldfish is rounder than the male's and tends to protrude from the body slightly as breeding season approaches.  When viewed from the side, the vent may appear as a raised surface on the female's abdomen. In addition to a protruding vent, the female's anal fin may appear slightly thicker than the male's.

Summary in Chinese:
雌鱼身型更为浑圆。 观察鱼的肛门。