The only prohibitions I know of in CA, are federal parks. But even then, you may get a ranger who won't stop you, but will tell you to "turn in all coins older than 50 yrs. old" (and then it's up to your math skills, right?

)
As for state-owned parks, you can get different answers. Ie.: ask enough bureaucrats high enough up the ladder, and what do you
think the answer would be?

I've hunted state parks (and especially beaches), right in front of rangers and anyone, and never heard "boo". But oddly, if you read the book "Treasure Laws of the United States" by a fellow named "Grimm", CA state parks sound discouraging. Or likewise if you go to the FMDAC's list of states, it will say that you
can detect in State of CA parks, AS LONG AS YOU ASK AT EACH KIOSK (or something to that effect). But by doing that, you merely leave yourself up to the whims of whoever's on duty, who can simply say "no", for
no other reason that he thinks you're a geek with a shovel (when in fact he probably would never have even given you a second thought or noticed you).
But on the city and county level, there are no prohibitions anywhere in southern CA. Is that to say you might not get a "busy-body" who tells you otherwise? Of course not. So you just go at low traffic times, be neat, don't go during an archaeologist convention, don't be in the middle of retrievals when busy-bodies are watching, etc... Odds are, no one cares.