How can you tell if gerbils are a girl or a boy?

The female gerbil has a distinctive genital shape (which doesn't include a sac like the male). If this doesn't help you can consider there visible nipples (they have six) on there belly BUT males have the same thing they seem to have less though. If you have both a male and female consider size difference (females are much slimmer/thin compared to a healthy male. A big clue is the yellowish color on the genital. The genital area will be very yellow/orange this is because they have there menstrual cycle. The color is much more visible compared to the male.