28 CFR Part 35 Title II Notice of Proposed Rulemaking (NPRM) (published 2008)

(f) Inquiries.  A public entity shall not ask about the nature or extent of a person's disability, but can determine whether an animal qualifies as a service animal.  For example, a public entity may ask: if the animal is required because of a disability; and what work or task the animal has been trained to perform.  A public entity shall not require documentation, such as proof that the animal has been certified or licensed as a service animal.