[Osage Orange]

People - including probably Heller - will keep his gun in his home without a trigger lock. Someone will defend himself, and the police will arrest the homeowner (or more likely, apartment renter) for not having had his gun locked.

How would they prove that?

[Dionysius]

So long as someone has a lock, they can’t prove the gun wasn’t first locked and them unlocked just in time to deep six the perp.

[Phlyer]

People - including probably Heller - will keep his gun in his home without a trigger lock.

How would they prove that?

Well, the cynic in me wonders why you think they'd need to prove it? They could arrest the guy and the burden of proof would be on him that he had, in fact, had it locked and managed to get it unlocked in time.

If he could show that - with an evidence trail of the lock sitting by the gun, and a demonstration of how quickly he could get it unlocked - then I expect they'd charge him with something because if he had that much time he should have just retreated from the apartment. The so-called "Castle" doctrine is only considered a legitimate justification for use of lethal force in a few states, so he'd have to show he did not have a reasonable chance to retreat yet did have enough time to get the lock off.

And all this before a DC judge and jury.

I expect he'd get convicted.