Just started to pick up prebans as I was going to get some ammo today. Looks like I learned my lesson early about these. I doubt the store will take them back in exchange for some full metal prebans they had.
Take a sharpie and color over any areas that seem suspicious or the whole top half of the mag if you have time to kill and let it dry.
Take the mag in and out a few times and see where the sharpie had worn off.
Sand area and repeat as many time as it takes to knock off the high spots.