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You will probably get more help with this if you post it in the breeders section but I hope the information below helps.
There are a few ways to tell if a dog is pregnant and each is done at a certain time after the tie. There is palpation, ultrasound, xray and hormone testing. The most common are palpation and xray. Xray shouldn't be done til after day 50-55 (depending on what the breeder and vet prefer). Palpation is cheapest but sometimes that is difficult for the vet because they can't get a look at the size of the pups.
She may be pregnant but it could also be a false pregnancy.
Happy whelping and don't forget to set aside at least $500-$2000 (depending on your area) for a possible c-section.