Can people in NY State own 30round mags?

Affirmative. So long as they are "pre-ban". Our ban mirrors the Federal AW Ban which expired on 9-13-04 with the major difference being that ours has no sunset.

Quoted: Can people in NY State own 30round mags?

Yes, but no. You can only have magazines over 10 rounds that were made prior to September 14, 1994.  

The only possible legal way around this to get newly manufactured mags is to buy new magazine bodies. But the catch is you must have older mags. Destroy the old mag body, and insert all new parts into the new mag body. You must also keep the old destroyed mag body. In a nutshell you cant end up with more mags than you started with.

Can these pre-ban mags be shot from post ban AR'S ?

Quoted: Can these pre-ban mags be shot from post ban AR'S ?

Yep!  The mag has to be manf'd before the 94 National AWB kicked in.  Basically if it is not date stamped or marked for LEO only you should be good to go.  Unless it is a manf that wasn't around before the 94 ban.  There are tons of pre-ban mags for sale on the EE.

How do you know if they are pre 1994?
I have a few date marked AR Mags but most of my 30s have no dates

no date is assumed pre 94 i belive

Quoted: How do you know if they are pre 1994? I have a few date marked AR Mags but most of my 30s have no dates

It's tricky, no date is not necessarily legal, at least not anymore.  

One of the problems with the Federal ban expiring and NY's going strong is there is a flood of newly manufactured, unmarked magazines on the market in free states.  Technically, without a date, it was assumed they were pre-ban, but now that assumption means nothing as people can get boatloads of mags without markings that were manufactured yesterday.

You at least want packaging with a date or batch number to confirm year of manufacture.  I bought a bunch of 1991 30-round Center Industries magazines from Magman in the EE.  They have no stamping on the mag body, but the package they came sealed in cleary has contract number and date of manufacture, and C.I. is still in business, so if it comes to it I can have a written statement from them regarding the date my mags were made.

There are plenty of real pre-ban mags available, so you can be choosy about which ones you want.  I only go for mags either sealed in date marked packaging, or with dates stamped on the mag bodies.

As I was told here not long ago, don't assume a mag is legal because it is unmarked!