Disclaimer: Not that I'm recommending anyone break the law...
Primarily enforced by the State, specifically the Department of Alcoholic Beverage Control (typically referred to by ABC).
Technically, a restaurant without a liquor license is not allowed to allow BYOB, but since the primary punishment is to lose their liquor license (or have it denied), it's typically winked at by restaurants that don't intend to apply for a license. Plus, as long as the patrons/owners are discreet, it's really hard to even detect, much less prosecute. A restaurant would have to have an explicit corkage fee on the menu or have some other blatant indicator (one restaurant I used to go to had "For beer and wine, visit the Liquor Store next door!" on its menu), otherwise the owner could just shrug his shoulders and say "I didn't know they were BYOB'ing". The only time a restaurant is really vulnerable is when they've applied for a license but it hasn't yet been granted. But someone has to specifically complain to the ABC for it to even notice, and then its power is mostly in being able to deny a liquor license.
There's also a very, very small number of ABC investigators (IIRC, four for the entire state), most of whom are busy investigating sales of alcohol to minors by convenience/liquor stores. I've never heard of the ABC staking out a restaurant without a license that allowed BYOB.
I'm inclined to think it's OK to bring your own wine into a restaurant anywhere in the state. I've done so many times over the years in both Northern and Southern California without a problem. It is always wise to phone ahead. Some restaurateurs do not allow the practice, as is their right, or have certain restrictions on what can be brought in. It's always good to know the corkage fee beforehand as well.