The 1994 Federal law (AWB) included a requirement for all "oversize" mags made after the ban to be marked with the year of manufacture.
So, basically, any unmarked magazine had to be considered as "pre-ban", without clear evidence otherwise (such as a model not made before 94), there was no way to prove its age.
The NY law bascally mirrored the Fed law, without the sunset provision. You'd have to check the exact wording of the NY law to see if it contains the same dating requirement, I don't personally know.
After the Fed law sunset in 2004, there was no legal mandate for magazines to continue to be marked with the year of manufacture, and some are not, although some makers have continued the marking. SO, you can have a mag from before 1994, unmarked, or after 2004, unmarked, but any mag legal for sale to civilians made between 1994-2004, would be marked. One also finds mags marked "for law enforcement and military only" (or something like that), legally for sale today on the civilian market.
Again, you will have to check the specific wording of the NY law to be certain which requirements are still in place.
Also check to see if there is any provision for allowing banned mags to enter the state. It could be treated as a separate issue.