This may sound a bit disgusting, but you must look in their private area. If it is a girl. you may see small nipples along the stomach. if it is a boy, you will see a penis in it's private area. I...
Until the babies open their eyes, it's difficult to tell gender, but not impossible. Simply lift the tail and take a good look at the two openings. Compare the babies to each other to get a feel for...
1. You may be able to find a small bump that is the male rat's penis. 2. You won't be able to find any nipples.3. Male rats tend to be a lot larger than females.