I would imagine they would enforce it any time they arrest someone who is in possession of a >15 rd mag, either by getting them to confess that they obtained the mag after 7/1/13 or by asserting that they obtained the mag after 7/1/13 and then putting the defendant in a position of essentially having to prove his innocence (and yes, I know the law explicitly says that the burden of proof is on the prosecution but that doesn't prevent a zealous prosecutor from going ahead and charging the offense anyway.)
They could also enforce by 'sting' operations, i.e, try to get people to buy or obtain >15rd mags through online sales, gun shows, etc.
Hell, there are gun shops right now that sell >15 rd mags in disassembled form as "repair kits" or sell them with the stipulation that they must be taken out of state or used for LE or military use only (as if military people buy their mags at a LGS.)* So they could start by cracking down on them.
In fact, that would make a great headline, wouldn't it? I can see the Local News scare headline now: "LOCAL SHOP SELLS ILLEGAL CLIPS AND COPS DO NOTHING TO STOP IT!" Great opportunity for some enterprising DA to get his name in the paper by busting these flagrant violators of state law.
*(And before someone asks, no, I'm not going to tell you which gun shops do this.)