Sexing multimammate mice (Mastomys natalensis) can be done by observing their genitalia and the presence of nipples. Males have a bulge at the base of their tail and their genitalia is further from the anus than in females. Females have 18 nipples, making them easily distinguishable from other rodent species.
Here’s a more detailed breakdown:
Female
Have 18 nipples, which is the most of any rodent species.
The distance between the urinary and anal openings is smaller than in males.
Male
Have a bulge at the base of their tail, which can be observed when they are held gently.
Their genitalia (urinary and anal openings) are further apart than in females.
Testes are rarely obvious, even during breeding season.