How do you know if your guinea pig is pregnant or just fat?

If you weigh your guinea pig(s) regularly, than if she is pregnant, she would have a great increase in weight.

For the many people who do not weigh their guinea pigs regularly, a simpler way is to simply watch her sides. When guinea-pigs are pregnant, they show a very definite bulk at the sides, particularly when they are picked up (gently) and held vertically (with their back legs being carefully supported). Both sides of the pregnant female have a slight bulge which is different from their normal weight/guinea-pig fat.