Summarize the following:
The most common symptom of a miscarriage is bleeding from the vagina. This bleeding can be slight or dramatic, depending on how big the fetus was and how many fetus' were miscarried. Even bleeding may be hard to spot, as many dogs will clean up the blood themselves before you spot it. However, if you know your dog is pregnant, keep an eye out for bloody discharge. If your dog is pregnant, its belly should grow at the puppies develop. If the belly of your pregnant dog is not growing, it may have lost one or all of its puppies.  A dog that has lost a pregnancy may even lose weight instead of gaining it. The usual gestational period of a dog is 63 days. Your pregnant dog's belly should grow dramatically during this time. There are some reasons that your dog may miscarry. If your dog has any of these issues, then it is more likely that she may have miscarried. Some factors that may induce a miscarriage include:  Infectious diseases, such as Salmonella, Brucella canis, canine Herpes, Parvovirus, or Toxoplasma. Trauma Uterine infection Hormonal problems Medications that promote miscarriage, such as chemotherapeutic drugs, chloramphenicol, oestrogen, or high dose steroids If you suspect that your dog may have lost a fetus, you should have it looked at by a veterinarian. Your veterinarian may elect to do an X-ray or ultrasound exam to look at the fetus' developing in the dog. If your dog has already had an X-ray or ultrasound to document the pregnancy, you should be able to assess whether all of the fetus' that were in the uterus are still present.

Summary:
Look for bleeding. Pay attention to the rate of stomach growth. Assess whether the dog has risk factors or medical issues that could cause miscarriage. Get an ultrasound or X-rays.