During the abdominal palpation, the vet feels the bitch's tummy for the presence of puppies. The earliest date that this is likely to be successful is at least 21 days after mating. At this early stage the vet is feeling for an enlarged, thickened womb rather than puppies (which are like small blobs of jelly at this point).  The best time for abdominal palpation is day 28. This is because the puppies are now big enough to make a "bump" in the womb, much like beads strung along a necklace. By day 35, the each puppy is surrounded by his own fluid capsule and the vet feels something akin to a fluid string of sausages. Bear in mind that at 28 days each puppy swelling is a maximum of 25 mm (1 inch) in diameter, making it difficult to differentiate embryos in a womb from food in the gut or feces in the bowel. Furthermore, if your dog is tense and braces her belly wall, it can be impossible to feel anything worthwhile–the equivalent of trying to see through a window that has been painted over. Likewise, a single puppy can easily be missed, or mistaken for another structure such as the bladder, a kidney, or even feces in the bowel. Don't feel too frustrated with the veterinarian when she suggests a pregnancy test, as the abdominal palpitation can be inconclusive. The blood tests looks for high levels of the pregnancy hormone, progesterone. To be reliable, this test should be done after day 28. Before this it's possible to get false positive results, because the high levels might be a result of delayed decay of the hormone present during ovulation. However, after day 28, high levels of progesterone confirm pregnancy. Ultrasound scans are a popular way of confirming pregnancy, because in the hands of a skilled operator small changes in the womb can be detected as early as 18 days. The other advantage is that the technician may be able to give you an idea of how many puppies are in the litter, which is useful to know when it comes to the birth, or whelping.  Placental units can be visible from day 18, and from day 28...

TL;DR in Chinese:
透过腹部触诊寻找怀孕的最初迹象。 了解腹部触诊未必准确。 第28天后可以验血，看看孕激素是否增加。 用超声波检测早期怀孕。 了解不建议照X射线的原因。