You can get ID tags for your dog identifying her as a service dog here
http://www.doggydmv.com/service_dog_tags.htm
You can get a certification and Photo ID Tag for therapy and service dogs here.
http://affluent.net/sara/sara4.htm
Neither one of these require any type of proof but the second site has this disclaimer:
Certification or registration through SARA does not imply, infer, suggest or guarantee that an individual animal or a disabled individual is qualified under any law or legality or is entitled to special considerations based upon any law or legality.
Not saying this is right or wrong, just letting you know this is out there.