How do you tell when the guinea pig sow is going to have babies or is due to?

It sounds like she is due any minute. Gestation is about 59 - 72 days. She will probably have the babies during the night when it is quiet so ensure there is a lot of nice clean bedding and fresh water and food - include lots of veggies and hay. Please do not handle her at all before she gives birth. Good luck - baby guinea pigs are so cute