Here is one link and a paragraph from the page.
What Makes an Animal More Than Just a Pet? Service Animal Information for People with Disabilities.
The Psychiatric Service Dog Society advises anyone obtaining a PSD should get a letter from their doctor. Their advice probably holds true for all service animal owners, especially those living with other types invisible disabilities. "This letter should be kept on file, in case you end up in court regarding your service dog, need to request ‘reasonable accommodation’ in the workplace, or a waiver of the ‘no pets’ rule at your apartment complex, etc." (PSDS, FAQ).
However, they advise against using this letter to gain access to public places, because business owners should not be mislead into believing such proof is required.