The easiest way to tell the difference between male and female guinea pigs is by measuring the distance between the guinea pig's anus and its genitals. According to Pet Informed, a male guinea pig's genital area measures further from the anus than a female guinea pig's genital area.
Another way to determine the sex of the guinea pig is by looking at the shape of the genitals. Pet Informed explains that the opening of the genitals on males looks like a dot and the opening of the genitals on females looks like a vertical slit.
According to Cavy Spirit, one way to sex guinea pigs is by carefully parting the opening of the genital area. Place the guinea pig on its back, and gently part the genitals. On female guinea pigs, the genitals look like a "Y" when opened. Male guinea pigs do not have "Y"-shaped genitals.
Male guinea pigs can also be sexed by gently pressing on the area above the genitals. Cavy Spirit reports that when the area is pressed, the penis slowly protrudes.
Additionally, male guinea pigs have a donut shape around the rectum area and females do not. Looking for the donut shape around the rectum is a quick way to determine the sex of a guinea pig, according to Cavy Spirit. However, Cavy Spirit advises that the best way to determine a guinea pig's sex is to look at its genitals.