Carefully pick each one up and examine its underside. On the females, you will be able to see two rows of nipples through the fur. The males will not have nipples. You can also sex by anogenital distance, which is the distance between the anus and the genitals. In the male, it is a long space, while in the female it is much shorter.
At this point, you can't tell from the presence of nipples, and the males have not developed prominent testicles yet, so to the inexperienced breeder they appear almost identical in gender. At this point, one of the only ways to sex them is to keep picking up two babies at a time and comparing their genitalia until you find two that are different. The one with the longer space between the genitalia and the anus will be the male. The shorter space will be the female. Unless you absolutely have to sex them at this point, it’s easier to wait until the testicles have descended on the males, which is sometime between 4 and 6 weeks.
Once they become sexually mature, the males will have fully-developed testicles, which are found at the base of the tail and can be easily distinguished from the females which have no testicles.
