Here are some general guidelines, but they're not foolproof:
* Males tend to be larger and heavier than females. However, this difference can be subtle, and some females can be larger than some males.
* Males have a larger distance between their anus and genitals. This is often referred to as the "distance between the bumps."
* Females have a small, round, raised vulva. It's located just in front of the anus.
* Males have a penis, which is not always visible. It can be hidden by fur, especially in young or less developed males.
The best way to determine the sex of a guinea pig is to have a veterinarian or experienced breeder check them. They can be more certain about their sex after examining the genitals closely.
It's also important to note that:
* Baby guinea pigs (under 3 weeks old) are very difficult to sex. It's best to wait until they are a bit older.
* Some guinea pigs may have unusual physical characteristics that make it difficult to determine their sex.
Remember, even with these tips, it's best to seek professional help for a definitive answer!