If a police officer takes a breathalyzer reading of someone, what kind of record proves to a court what the reading was? General US information and/or info about California are both appreciated.
Different states vary on all of this stuff but many times the roadside breathalyzer is given to establish probable cause to arrest a driver. After that the driver is given another test on a computer-connected breath tester at the station (or wherever). It's these big testers that are usually used as the evidence and are the ones which are at the center of controversy in various states.
As @Dale M stated, for roadside tests the officer can write the results in a notebook, some breathalyzers have printers, and other are connected to a computer. Whatever the case, there is usually a process that officers go through to ensure this stuff is accurate and complete. A lot of it is departmental policy and would only be revealed if challenged in court or perhaps via an open data request. But again, this record is to show probable cause for the arrest and the trip to the bigger badder test which is fully computerized and documented and witnessed. (In at least one state (NC) you have the right to have a witness present when you are tested.)
EDIT: Oh, and if your plan is to challenge probable cause because the record-keeping on the roadside breathalyzer is mediocre, the cop can cite all sorts of other probable cause, like he saw you swerving, you were slurring your speech, he smelled alcohol, you failed the in-car test, and you failed the roadside walking, touching, dancing, singing test.