Well it works a bit differently here in California. For as long as I have owned guns here (18 years old), you have to pass a background check. Background check application includes the serial number of the gun and if you pass and the gun is sold to you, the California DOJ records the ownership of the gun under your name. I had a LEO run my license once and heard the dispatcher tell the cop I own firearms. I'm pretty sure it wouldn't be too difficult for them to find out the date of purchase.
Even private sales here have to go through a licensed dealer. So you you go take the gun to a dealer, he does all the background shit, collects your money and pays you after the waiting period has passed and the buyer picks up the gun. The only lawful transaction here without a dealer is gifting guns to immediate family members but I believe that is rifles and shotguns only.
Anti could probably comment better on this since he's a cop in So. Cal but I bet you if they found you with an extended magazine, it would be a pretty simple task to run your registered gun purchases and find out your date of purchase.
Even then though, you are innocent until proven guilty. So it wouldn't be up to you to prove you bought them before the ban. It'd be up to the prosecutor to prove you bought them after. So yea, I think it'd be tough to prove but I'm pretty sure it's a felony charge so why risk the hassle of the arrest record and having to fight it in court.
