Bryan,
I'll just pass along some info about the area I live in (northern California, south SF Bay area).
State parks can be hunted with specific permission from the park office. Usually they limit you to certain areas, but will let you hunt.
Santa Clara County (basically Silicone Valley) park hunting requires a permit. I have one.
San Jose city parks (tons of em) require another permit, even though SJ is in Santa Clara county. I have one.
Beaches (Santa Cruz, etc) don't require permits.
I live in a very foo-foo town (Los Gatos). I checked the local ordinances on the web and found nothing forbidding hunting in LG city parks. However, I stopped by the police station and had a friendly chat with the police chief...explaining code of ethics, etc. He said..."no problem".
I think the keys to being left alone are:
1. Leave the place better than you found it...see my post in the Featured Threads at the top of the main forum page.
2. Be friendly with the groundskeeper folks. You may have a permit/permission/etc, but if they want to cause difficulty...they can.
3. Avoid complaints from other park visitors. For example: Don't start detecting in the middle of a family gathering...return the next day for their loot.
4. Dress nicely in parks. Stupid as it may sound, if you are detecting around a soccer mom with a couple of rug-rats looking like you just crawled out from under an overpass...you're gonna get complaints.
All that said, I've gotten some of my best local intel from park rangers, groundskeepers, and even visitors. All you gotta do is act civil and most folks will treat you well. Of course, there's bound to be a "one bullet Barney" mixed in, but just smile and remain courteous.
John