Vaccination records are the only way to know, for certain, whether an animal has been vaccinated. Veterinarian clinics maintain records of each of their patients' vaccinations. In the event that the animal was obtained from a rescue organization, the new owner should receive copies of these files from the rescue organization.
Veterinary confidentiality laws vary by state; however, if a previous owner or rescue organization is unable or unwilling to provide proof of vaccination, it may be possible for a new owner to obtain her pet's vaccination records from the animal's previous vet.
If the previous vet is unknown, unreachable or unavailable, the owner can request that the new vet run blood titers. Blood titers are dot ELISA assays that can be used by a vet or veterinary practice to determine whether or not an animal has antibody-driven immunity to a certain disease. Unfortunately, blood titer results are not always accepted as alternatives to vaccination records because they do not determine, for certain, whether an animal has been vaccinated. In cases where proof of vaccination is required, such as rabies vaccinations, blood titres are not accepted. In such an instance, the animal has to be vaccinated regardless of its blood titer results.