Update!! This is the best photo I can get of my son and Belle since they wouldn't sit still... but shes been home since Christmas day and I couldn't be any happier
One way to establish who is the rightful owner, assuming the dog has been a regular vet patient, would be for both claimants to go to that vet or vets with the dog to see who the vet vouches for. I know my vet knows my dog really well.