Petfinder.com requires shelter or rescue organizations to verify that they are legitimate before signing up and listing available pets on the site, so if they are registered, they are probably legitimate. Petfinder.com has specially trained shelter outreach employees to verify each new group.
If you come across a rescue on Petfinder.com that seems suspicious, you can contact Petfinder through the website and ask the organization to investigate. There are several possible warning signs that a rescue may not be legitimate. Rescues that deal primarily with designer breed dogs or a specific popular breed often warrant some more investigation, because sometimes puppy mills pretend to be rescues to profit off of dogs they cannot sell.
Legitimate rescue groups should generally only release dogs that are spayed or neutered, fully vaccinated and have been checked by a vet for general health. If they do send a dog home without these measure, they should be able to give a clear and sensible reason for it.
Communication is another way to spot a scam rescue. Although many good rescues are busy and may take a little time to return calls or emails, be wary of ones that seem unable to communicate in a timely or reasonable manner. A rescue should be able and willing to answer all of your questions, and ask you many questions in return. Be wary of rescues that do not seem curious about your ability to care for the dog.