I have seen many people using metal detectors on Southern California beaches. I imagine the rest of the coastline isn't much different in that regard. As far as I know, and unless there are rules prohibiting this, beaches like Newport Beach, Huntington Beach, Corona Del Mar, Laguna Beach and Long Beach as well as Aliso could all be potential beaches.
Ideally, for very long stretches of sand where you can walk and walk, I would say Huntington and Newport would be among the two best of those beaches. Of course, always check the rules of said beaches before doing so.