They knew I had 17 shot mags because they were the only kind that were sold until now, apart from the 33 shot mags. That was based on trust though- they weren't really hunting for magazines, you were just trusted to hand all the old ones in.
The deciding factors were barrel length and caliber. 9mm OK- .40 and above, not ok. ~5" barrel ok- shorter, not ok. I also have to go to the range 14 times a year, and use both guns at some stage, in an organised competition. This is recorded on a card that is returned to the police at the end of each year. If I don't do that, then they figure I don't really need them for sport after all and I have to dispose of them.
I had to take my guns to the police station where the barrel length was measured and the calibers verified, even though they were already registered (handguns have been registered in Australia since the 1930's). People with .40 caliber glocks, compact pistols, 1911s, and so on, had to sell them to the government, for a fairly good price.
I also have to prove why I 'need' more handguns, previously you could just have as many as would fit into your safes

. Now they say 4-5 is enough, of different types ie a .22, a revolver, an air pistol, a black powder revolver, and a semi-auto.
One part of the law that you would probably find strange in the US is that the police can come to your house at any 'reasonable time' to inspect your guns, without a warrant. I think they need to make an appointment, they don't always seem to though. In reality they will only do this if someone complains about you shooting around your property or something like that.
All the guns that were sold to the government were melted down, the whole excercise cost the government $100 million and destroyed about 60,000 handguns across Australia. About 25% of pistol shooters quit for good and sold ALL their pistols, even still legal ones, to the government.
I am not aware of anyone who didn't comply with the new laws, nor was this raised as a possible option by anyone.