Yeah, but it's easy to reseal mag bags if you're careful about opening them.
The secret is that you should see some light insertion marks on the mag body. Yes, on NIW mags! That's because they should have been run through a QA test guage, and there's almost always some evidence of that on the moly finish.
Look for obvious errors such as green followers in older mags (Sanchez started putting them in SOME of their mags in '88, and they were required for all USGI mags as of late '91) or worn/mis-matching floorplates. Also, if the floorplates have moly finish on them, other than maybe a few drips from the mag body, they've been refinished. Stock floorplates are anodized black, but have no other finish.
I generally recommend AGAINST buying refinished mags. The key reason being that even someone who did the best job on the refinishing job may have started with bad mag bodies, and it can be very difficult to tell once they've been refinished. If you want refinished mags, use the info in the tacked thread and refinish them yourself. Then you KNOW you have good mags.