” ‘...How can they know?...’
Start with the head stamp. If manufacturer didn’t ship to CA in that year, you are out of luck. ...”
Headstamps containing the date are found only on ammunition made to MIL STD only. US commercial ammunition bears manufacturer and caliber information.
Some foreign-made ammunition is stamped with otherwise unannotated numbers, but these digits do not always indicate the date of manufacture.
That is not the last word on enforceability headaches.
Many commercial reloading companies exist. Often, they obtain once-fired cases in military chamberings, which bear headstamps indicating original date of manufacture. They produce remanufactured ammunition using these cases. Would the law apply to original year of manufacture, or to year of remanufacture?
A lot of the cheap 5.56 ammo on the market today from Federal is overruns from the Lake City plant. ATK’s military contract overruns are on the market too. Both of these follow military headstamp protocols. If it’s being sold with an “XM” number, it has a date on the headstamp.