You may receive notice that you are required by the San Francisco Code to maintain a backflow pressure assembly device. The threat of contamination to your water supply is so serious that no state, municipal or water utility allows just anybody to test these devices. 

SFPUC requires that your cross connection device be tested by someone who has been trained to do the job right. After you have been required to have the device inspected, you can find a list of certified testers from the San Francisco Department of Health here.