Can I require proof a dog is a service/therapy dog?

[Amy Mueller]

I allow dogs in my rentals, but not breeds excluded in my insurance policy.  Rotts, pitbulls, Dobermans.  When I informed an applicant of these restrictions, she told me her dog was a pitbull, but a therapy dog, and she could get paperwork.  How would I know what paperwork is authentic and not something paid for online?  Am I even allowed to require documentation verifying the dog is a therapy dog?  Will insurance make an exception as well if the dog is a therapy dog?