How can I be sure my neighbor's new puppy isn't a parvo carrier?

Be sure to get them vaccinated on schedule, and keep them indoors until their vaccines are complete and the vet gives the OK. Make sure your neighbor keeps her dog out of your yard and better still, make sure she stays out of your house so that she isn't going to bring any parvo germs (if the pup has it or is going to get it) into your house on her shoes. Obsessive? Yes but parvo kills.

Keep them indoors till they have had all their required shots.

Here is a list of the shots a puppy requires:

6 to 7 weeks of age: Administer first combination vaccine (Distemper, Hepatitis, Parvovirus, Parainfluenza, Coronavirus).

9 weeks of age: Administer second combination vaccine.

12 weeks of age: Administer the third combination injection and possibly a lyme vaccine inoculation. Generally a lyme vaccine is repeated two weeks later, then once a year.

16 weeks of age: Adminster the last combination vaccine.

12 to 16 weeks of age: Adminster rabies vaccine. The timing of this vaccine may depend on the laws in your area (since this can be a human disease, too). Confirm with your veterinarian and check your local and state laws.

Talk to your vet when it's safe to finally take them outside.

Keep your puppies away from her puppies until they are all vaccinated.