
I guess that it will depend on breed, but I find it really is easy to feel any that are left. And I don't prod and dig around, I find them easy to feel.
I once had a girl that had stopped contracting after her fist pup was born, and ended up going in to the veterinary hospital for an oxytocin jab. She produced a pup after one jab, then stopped. Eventually a second jab was given and she had another. And stopped again. It wasn't a vet that I was used to, and he seemed happy that she had finished.
I told him that I could feel another pup. On examination he told me that he thought it could be the spleen. Glad I was insistant, as he felt that he had to do a scan to prove me wrong. First we saw a spine, and then a hearbeat! :-)