As the babies grow in size, you will actually be able to feel their bodies and even estimate how large of a litter she will have. When the babies start kicking and moving inside her womb, she is roughly three weeks away from giving birth, give or take a few days. You won't get many warning signs as the birth day approaches. In short, there's really no way to predict when she will go into labor. Fortunately, you do not need to do anything for her to prepare for the babies, other than make sure she has plenty of soft bedding to nest in.
Most births tend to take place around dawn or dusk, which is when guinea pigs are most active.
If you are fortunate enough to be present for the birth, you can watch quietly without disrupting the process. If the male is still with her, and he hasn't been neutered, then you should remove him immediately, as the sow will come into season within an hour after birth. Babies are typically born five minutes apart, and the entire litter should deliver within a half-hour. If it goes on any longer than this, it could mean that she is having trouble giving birth, and you will need to get her to a vet right away. With each contraction, the mother will push the babies out, one at a time.