This would be almost impossible in most states like here in Maine. How are they going to come and take firearms that they don't even know that you have? Since we don't register them they have no idea who has what or how many.
Data mining. They can tell from a combination of looking at 4473 records, credit card purchases and other info that you probably have guns. In theory, someone who has generated lots of 4473 records and made lots of purchases at gun stores could actually indeed own zero guns. But, we know that would be rare. And, the people doing the confiscations would take great pleasure in tearing your house apart right down to the studs, and beating the crap out of you while they do it, that they'd be okay with the occasional miss when going door to door. Besides, if they find no guns, they will assume you are just hiding them away from the home (even if you really aren't!)
Remember, it isn't about getting every last gun off the streets. It is about the tyranny... that's the end goal, and can readily be accomplished while leaving a good percentage of guns still out there.