Help sexing babies?

I recently rescued a very sweet female dwarf dumbo rat (along with her sisters) & it turned out she was pregnant! She had 7 pups but sadly one didn’t make it. The other 6 are growing nice and healthy & I’m trying to figure out their sexes. I know females have more nipples than males but I can hardly notice any nipples at all lol. Also, when should I separate them by sex so they don’t inbreed?