When you enter California by road you are usually subjected to an agricultural inspection by state authorities. Every highway into PRK has an agriculture inspection station, and they are all usually in operation.
They typically ask if you have any fruits or vegetables or live plants. I wish I had a picture of the face of the inspector who had me open the moving van in which I imported in a Ryder truck my now-ex's stuff from South Carolina. We had a big mess of Spanish moss (Tillandsia usneoides) stuffed into the back of the moving van.
The inspector had NO CLUE what it was. She was from Fresno and had never heard of epiphytes. When I told her it typically grows on oak trees she suddenly had an angle to work. She pawed through the stuff looking for known parasites that would infest a living oak tree. After about 5 minutes she srugged her shoulders and waved us on.
The Border Patrol (federal Immigration and Naturalization Service officers) also maintains checkpoints on highways that lead into California. Compared to the state's agricultural inspection stations, the INS stops are poorly located and underfunded. Personally I believe that stopping everyone on the highway is unconstitutional, but their job is to find international smugglers of people, counterfeit shoes, drugs, etc.
On the pretext of searching for bad oranges or heroin, these people COULD search your whole vehicle. If they found high-capacity magazines they might check to make sure you aren't carrying any loaded weapons. Other than that, they have no mandate to enforce the ban on importing high cap mags.
The California Highway Patrol might be inclined to question the legality of your magazines, but only if you are being an ugly tourist.
Smuggling high capacity magazines into PRK is illegal. Don't do it.