Service Dog badges.

Until you get some service animal badges printed, print out some copies of this to show to anybody that questions you when you say you have a service dog:
Commonly Asked Questions About Service Animals in Places of Business

Especially point out the part that says:
However, an individual who is going to a restaurant or theater is not likely to be carrying documentation of his or her medical condition or disability. Therefore, such documentation generally may not be required as a condition for providing service to an individual accompanied by a service animal. Although a number of states have programs to certify service animals, you may not insist on proof of state certification before permitting the service animal to accompany the person with a disability.