Sex genotyping of mice
While adult mice can be readily sexed using anatomical features, young male and female mouse embryos are not yet sufficiently developed to assess their sex. In fact, their genital tubercles look superficially identical. At E16, only about 4 days before birth, the tubercle looks like this: E16 genital tubercles in Baskin's book, p. 108, figure 2.
If the sex cannot be determined by inspection, a genotyping PCR for any region only found on the Y chromosome  can be used.