On first glance, male and female guinea pigs look similar. They don't have obvious differences, such as a mane on the males, or different colors. Their genitalia are also unobtrusive. To reliably determine the sex of a guinea pig, you need to pick it up and take a much closer look. If you are still unsure, ask your vet during the routine checkup that all new pets should get.
Pick up each guinea pig. Turn it upside turn, holding the animal securely around its chest and shoulders. Better yet, sit on the floor so the animal doesn't fall far if it wriggles free.
Examine the genital area. Mature males often have visible, doughnut-shaped testicles. These might not be apparent on younger males, however, and an absence of obvious testicles does not mean the guinea pig is a female.
Note the distance between the anus and the genital opening. The anus is the opening nearest the rear of the animal. They are closer together in females. Females also have a clear Y-shape linking the two openings.
Press gently just above the genital opening. If the guinea pig is a male, his penis should come out.
Turn the guinea pig the right way round as soon as you have determined the gender. Guinea pigs are not comfortable being held upside down.