CA enacted a ban on buying new
ones (buying, importing, manufacturing, selling, etc) after
the date the law took effect, but allowed individuals to keep the existing ones owned before
Yes, the burden is on LE to show any hi-cap mags in an individual's possession were obtained after the ban took effect. (Think catching someone in a "street corner mag deal", with money & mags changing hands ... buying/selling at gun shows ... selling them across the state line, etc).
Also, being in possession of a new hi-cap mag that was manufactured after
the law took effect would be hard to explain, as would possessing one for a pistol that wasn't even in production before the ban took effect.
I've been also been told by at least a couple of gun companies (during armorer classes) that in some cases they might be able to help LE identify the production period of some magazines (due to manufacturing changes, markings, vendors used, etc).
Only if it's unlawful for that individual to possess that firearm.
CA allowed existing owners of what were considered "assault weapons" to register them with the state so they could continue to own them. Naturally, this didn't apply to persons who were prohibited from owning/possessing firearms, or persons who owned firearms that were already unlawful to posses under other laws. (There was some confusion, to put it mildly, about extensions of the original registration period
... . but that's a story for another day.)
If someone lawfully possesses what CA has defined as an "assault weapon", then any cop seeing one in someone's possession during a traffic stop (or during any other situation where the cop has lawful reason to be, and see what he/she suspects may be an "assault weapon"), can easily check the serial number against the state's DOJ database and confirm either lawful ownership or unlawful possession. (Yes, if the computer system is down at that particular moment, it may complicate or delay things a bit.)
Also, registered owners of CA assault weapons receive an original registration for the specific weapon from CA DOJ, which they can show to LE.
Transportation of registered assault weapons is a bit different than transporting other long guns. http://caag.state.ca.us/firearms/travel.php
If someone missed the deadline to apply for a registration for their weapon several years ago, or simply decided not to do so (for whatever reason), then their weapon isn't lawfully registered to them with the state as an "assault weapon" and would be considered contraband.
The "off-list" lowers and assembled rifles made with "bullet buttons" have been selling pretty strongly in CA, but that category of semiauto rifles may be addressed by legislation at some point. Dunno if/when, or how it will be worded and implemented. I'm retired, and don't own an off-list rifle, myself, so I haven't exactly been keeping up on it.
I know a number of guys who have been buying off list lowers, though, so I imagine I'll hear further info if/when something is passed and chaptered into law.
Some further FAQ info regarding CA law and "assault weapons". http://oag.ca.gov/firearms/regagunfaqs#1