Meant to add an ultrasound or x-ray can be done. A good Vet can usually tell by palpating.
Has she been off her food?
Nipples enlarges?
Vulva swollen?
Possible white to straw-colored discharge from the vulva.
Hope you are prepared well in advance.
Was she wormed and up to date on all her shots before being bred? Puppies get some immunity from the mother.
Take her to the vet and have an abdominal x ray done. Puppy skeletons will show up on the x ray if she is pregnant. I used to work for a vet, and when we did these xrays, it was always fun to count the little heads to see how many pups there were. :-) - Blu