A property manager/landlord may require the tenant to provide written verification from the tenant's healthcare or mental health provider (when the disability is not obvious) that the tenant has a disability and needs the service animal (the provider need not be an M.D. – in fact, in the U.S. Dept. of Justice and Dept. of Housing & Urban Development have said that a medical professional, a peer support group, a non-medical service agency, or a reliable third party who is in a position to know about the individual's disability may also provide verification of a disability.) When property management/landlord requires proof that the tenant is disabled and that the accommodation assists the person with that disability, they still cannot require the tenant to provide information about the nature or severity of the disability. So yes, you can require a letter from a healthcare provider, that the service animal is required. Most disabled persons will gladly get you a letter that you can verify the need of the service animal.