Yes, you can know, at a certain point.
How old are these pigs?
First, have you turned both pigs over and confirmed that their genitals are the same? if they're not, she's likely pregnant, and you should separate them right away. She can get pregnant within minutes of giving birth, even if it's a premature birth. So the male needs to be out of the cage now. See (broken link removed) for some pictures for comparison.
If you've got two females, is there any chance that she's been with a male recently? If so, how long ago? Guinea pig pregnancies last about 9-10 weeks. If it's been longer than 11 weeks, she's definitely not pregnant.
If you don't have a kitchen scale to weigh them with, you need one. It's the best medical equipment you can have for your pigs. Right now, if you're weighing both of them daily (in the morning before feeding them), you can compare the rate they're gaining. If the chubby one's weight is increasing much more rapidly than the other, she may be pregnant.
It's very unlikely that a guinea pig is ever overweight unless it gets way too many pellets or has some mobility problem that keeps it from moving around. It would be like you being overweight if you ate nothing but salads without dressing.
Once a pregnancy is 5-6 weeks along, you can begin seeing definite signs, and can make some educated guesses about when the pups will come. However, they're can't read the schedule, and they'll get here when they get here. See
https://www.guineapigcages.com/foru...inea-pig-is-pregnant-And-how-far-along-is-she.