You will be feeling for slight movement or you may be able to feel the skull if the pup is near the surface. Gently place your fingers on her abdomen. If she has a large enough litter you should be able to feel some movement or hard lumps.
The best time to palpate is after the 45 day of pregnancy when individual fetuses may be individually felt.
If she is having a small litter, you may not feel anything.
I hope this anwers your question. Good luck with mom!
Her internal organs in the abdominal area will not be small, have a round surface, or be moving.
What breed of dog is she? How far along in the pregnancy is she?