As others have said, NY state bans handgun magazines that hold more than 10 rounds unless they were manufactured before the Sept. 1994. The problem is (as others have also said) that the most magazine don't have a date of manufacture on them.
In practice what that means is that while someone can probably come close to the manufacture date by checking with the manufacturer when specific features/styles were changed, many (probably most outside of NYC) won't prosecute solely for possession of a hi-cap magazine. Manufactures/dealers/stores etc. do not carry post-ban magazines, nor will they ship them into NYS. Magazines marked with LE-Only are a different matter, and I would think that since they are easier to identify, you'd be more likely to be charged if found carrying one.
As far as NYS assault weapons ban goes, it's overly complicated, and difficult to outline in a few sentences. Almost nothing is completely illegal, but it depends on when the rifle was manufactured (pre-ban or post-ban) and how many features from a list are installed. So you could have a pistol grip, or a collapsable stock, or a bayonet lug etc. and still be perfectly legal, but have too many of them on one rifle, and it becomes illegal.
I've found there is a big difference in NYS (excluding NYC) between what the law says and what is actually enforced. For example, besides the hi-cap mag issue, while it's illegal to carry a firearm onto school property, the LEOs I've talked with would not press charges on someone driving to school to pick up their children (stay in or near your car and don't enter the buildings) while CCW. I've found up state NY to be very gun-friendly.
