The gerbils must be of sufficient age to make a definite conclusion – that is, they must be sexually mature. If you hold the gerbil by the base of the tail to reveal the sexual organs you will notice two small lumps, one on front of the other. In the female these are distinctly closer together than in the male. It is hard to judge the sex unless you can compare male and female, but if your gerbils appear to look the same in this department they are probably of the same sex.