Two part question:
1. How is a mag determined to be pre-ban if the magazine was manufactured pre/post 1994? Mfg dates aren't generally stamped on magazines.
Some mags are indeed, date stamped. Sometimes there are identifying marks, but in a lot of cases there is no way to prove something is post-ban unless it's a captain obvious situation- eg, if you have something like an S&W M&P, then all the "large capacity" (those are the words in the law, not mine) magazines for those guns are clearly not possession legal in MA (outside of the LE/Mil exemption) because the M&P only existed as of a few years ago.
2. What does LE / courts look for to prove it is pre-ban?
There is not a lot of evidence on the ground in MA for this, but typically, what's been seen in the past is the DA will try to call the gun manufacturer, or possibly the magazine manufacturer, to get them to rat on you or give them info to convict you with. Thankfully most manufacturers don't cooperate with this kind of crap, or there is no manufacturer to call. .
Chances are if you are ever in this situation you will have bigger problems to deal with than the provenance of your magazines.
Just moving to MA from ME and trying to wrap my head around some of this. Thanks.
You basically moved from the free world to a place like Venezuela. Don't bother trying to comprehend it- in MA gun owners are basically treated as criminals with a card that grants us some level of protection/immunity against the state, that's about it. The magazine/AWB issue is basically mouse turds compared to the other problems gun owners face in this state, namely the fact that your ability to own guns is basically controlled by your police chief- and he/she can more or less take away your rights at the drop of a hat, without due process. That's pretty frigging scary, by itself.
I hope for your sake that some day you can move back to Maine, or some other free state.
-Mike